Circadian de(regulation) in physiology: implications for disease and treatment
- 1Center of Brain, Behavior, and Metabolism, Institute of Neurobiology, University of Lübeck, 23562 Lübeck, Germany;
- 2Charité–Universitätsmedizin Berlin, corporate member of Freie Universität Berlin, Humboldt Universität zu Berlin, Laboratory of Chronobiology, Berlin Institute of Health, 10117 Berlin, Germany
- Corresponding author: achim.kramer{at}charite.de
Abstract
Time plays a crucial role in the regulation of physiological processes. Without a temporal control system, animals would be unprepared for cyclic environmental changes, negatively impacting their survival. Experimental studies have demonstrated the essential role of the circadian system in the temporal coordination of physiological processes. Translating these findings to humans has been challenging. Increasing evidence suggests that modern lifestyle factors such as diet, sedentarism, light exposure, and social jet lag can stress the human circadian system, contributing to misalignment; i.e., loss of phase coherence across tissues. An increasing body of evidence supports the negative impact of circadian disruption on several human health parameters. This review aims to provide a comprehensive overview of how circadian disruption influences various physiological processes, its long-term health consequences, and its association with various diseases. To illustrate the relevant consequences of circadian disruption, we focused on describing the many physiological consequences faced by shift workers, a population known to experience high levels of circadian disruption. We also discuss the emerging field of circadian medicine, its founding principles, and its potential impact on human health.
Keywords
The circadian network
The Earth's rotation on its own axis creates recurring cycles of light and darkness that have led to the evolution of endogenous timing systems (circadian clocks) in most organisms to enable them to anticipate periodic changes and challenges in their environment. Light is a fundamental temporal cue (or zeitgeber, time giver) for organisms to synchronize (or entrain) their internal circadian clocks with the external environment. In mammals, although light detected by retinal photoreceptors in rods and cones contributes to the image-forming process, the nonvisual response is crucial for circadian entrainment and depends largely on the photopigment melanopsin, with some input from rods and cones (Panda et al. 2002, 2003; Hastings et al. 2018). Melanopsin is found primarily in a specific group of retinal cells known as intrinsically photosensitive retinal ganglion cells (ipRGCs), which are sensitive to blue light and convert photons into electrical signals. These signals are transmitted through the retinal hypothalamic tract to the suprachiasmatic nucleus (SCN), the master clock in mammals, which then processes this temporal information and transmits it throughout the organism via multiple pathways (Hastings et al. 2018). Neurotransmitters released from ipRGCs, such as glutamate and pituitary adenylate cyclase-activating polypeptide (PACAP), prompt the rapid transcription of clock genes in SCN neurons, including the Period (Per) genes (Hastings et al. 2018).
At the molecular level, the circadian clock is cell-autonomous and operates through transcriptional–translational feedback loops (TTFLs), a dynamic interplay of genes and proteins that enables endogenous rhythmic gene expression throughout the day. The proteins BMAL1 and CLOCK (or NPAS2) form a heterodimeric transcription factor complex that initiates the expression of Period (Per) and Cryptochrome (Cry) genes by binding to E-box sequences in their promoters. PER and CRY proteins are produced during the day and form the core of a multiprotein complex that inhibits CLOCK/BMAL1 activity after a delay of several hours, thereby reducing Per and Cry transcription during the night. This decrease in transcription and regulated protein degradation limits the repression so that the transcription cycle can begin again. The main loop is complemented by a secondary circuit that involves CLOCK/BMAL1 activating the expression of nuclear receptor subfamily 1, group D, members 1 and 2 (Nr1d1/2; also known as Rev-erbα/β) and group F, members 1, 2, and 3 (Nr1f1-3; also known as Rorα-γ). REV-ERBs and RORs modulate Bmal1 expression by interacting with ROREs in the Bmal1 promoter, exerting negative and positive regulation, respectively. A third feedback mechanism features the rhythmic expression of the BMAL1/CLOCK target gene Dbp (albumin D-site binding protein) coding for a PAR-bZip transcription factor. DBP binds to D-box enhancer regions, as does the repressor nuclear factor interleukin 3-regulated (NFIL3; also known as E4 promoter-binding protein 4 [E4BP4]). Together, they modulate the rhythmic expression of core clock genes such as Pers. For an in-depth view of the molecular underpinnings of the circadian clock, the readers are referred to external reviews (Takahashi 2017; Patke et al. 2020; Laothamatas et al. 2023). The circadian core oscillator regulates the rhythmic expression not only of many of its components but also of thousands of clock-controlled genes. Crucially, de novo transcription contributes to only about a quarter of rhythmic transcripts (Koike et al. 2012; Menet et al. 2012), suggesting that additional mechanisms such as chromatin, post-transcriptional, and post-translational regulations are also important for maintaining and generating circadian rhythmicity (Janich et al. 2015; Robles et al. 2017; Hurni et al. 2022).
The circadian clock network is a complex system that includes local clocks in each nucleated cell (individual cellular clocks), collectively organized within tissues (local tissue clocks), and integrated throughout the body (cross-tissue communication). The SCN plays a critical role in providing timing cues to peripheral clocks through partially redundant signals, including autonomic nerve output, cycles of body temperature, hormones (e.g., glucocorticoids and melatonin), and behaviors (e.g., food intake and locomotor activity) (de Assis and Oster 2021; Finger and Kramer 2021). Studies have shown that ablation of the SCN by targeted deletion of Bmal1 eliminates peripheral rhythms when mice are kept without light cues. However, rhythmicity persists under a light/dark cycle, albeit with slightly altered phases (Husse et al. 2014). Further studies in mice with functional clocks in only one or two tissues suggest that local clocks drive only a subset of rhythmic processes, highlighting the important influence of systemic cues on rhythmicity (Koronowski et al. 2019; Welz et al. 2019; Greco et al. 2021). For example, mice with a functional clock only in the SCN show both behavioral and metabolomic rhythms, underscoring the critical role of behavior in maintaining metabolic rhythmicity (Petrus et al. 2022). Recent evidence suggested that peripheral clocks gate systemic brain-derived signals that may be integrated or suppressed, thus contributing to rhythm generation in peripheral tissues (Kumar et al. 2024; Mortimer et al. 2024).
The growing recognition of the importance of the circadian system for health and disease led to the 2017 Nobel Prize in circadian research, awarded to M. Rosbash, M. Young, and J.C. Hall for uncovering the molecular mechanisms of circadian rhythms (Dibner and Schibler 2018). The circadian system is critical for the optimal functioning of organisms, orchestrating internal temporal programs and synchronizing them with the environment. In other words, the circadian system creates temporal windows for incompatible biological processes (e.g., DNA replication and UV light exposure) throughout the day and acts as a predictive system to anticipate recurring events in the environment. This is achieved by sensing external time cues (e.g., light through the retina), maintaining an internal timing system even in the absence of these cues (central clock in the SCN), and distributing timing information to peripheral organs.
Given the prominent role of the circadian clock as a regulator of physiology, it is not surprising that disruptions in the circadian clock have been linked to various diseases. This review discusses recent discoveries of how circadian dysregulation contributes to the onset, development, and progression of diseases, focusing on a population with significant circadian disruption: shift workers. We also discuss recent strategies to detect, target, and exploit the circadian clock, which are promising and leading to the emergence of a new field: circadian medicine.
Circadian disruption
The smooth temporal coordination of bodily processes with rhythmic environmental factors depends not only on the presence of external zeitgebers (e.g., light and food intake) but also on the proper timing of these cues. Conflicting cues can be detrimental to an organism because they can lead to incompatible processes happening at the same time. From an evolutionary standpoint, the living conditions of modern Homo sapiens differ significantly from those of our ancestors. The circadian clock evolved under conditions of food scarcity and seasonal variations in sunlight and temperature. In less than two centuries, however, our living conditions have changed dramatically with the advent of several technological advances, such as artificial lighting, heating systems, and the development of high-calorie foods and beverages that are available around the clock (Gerhart-Hines and Lazar 2015). In addition, lifestyles such as shift work, travel across time zones, inconsistent working hours, and nighttime exposure to artificial light are on the rise. However, these modern challenges were not present during the evolution of the circadian clock and make today's living conditions significant stressors for our circadian system (Gerhart-Hines and Lazar 2015) . In fact, there is growing evidence that these conditions are highly detrimental to the circadian clock and overall health (https://publications.iarc.fr/593).
The invention of the light bulb, and later the more energy-efficient light-emitting diode (LED), has profoundly changed human society, integrating light into almost every aspect of our daily lives. However, the blue light that is emitted by many LED devices can have negative effects on the circadian system. For example, disrupting melatonin production at night can critically impair the sleep–wake cycles (Blume et al. 2019; Walker et al. 2020). Experts are increasingly warning about the short- and long-term risks of LED light on the circadian system and health (Blume et al. 2019; Tähkämö et al. 2019; Walker et al. 2020; Moore-Ede et al. 2023; Zielinska-Dabkowska et al. 2023). To counteract this, the emergence of products that aim to reduce blue-light emission from LEDs in the late evening and at night or that offer adjustable color temperatures—collectively known as “circadian lighting”—promises an exciting solution. This innovation aims to harmonize artificial lighting with natural behavioral rhythms, potentially reducing future health problems.
Epidemiological studies have linked exposure to light at night (LAN), such as that experienced by night shift workers, with a higher risk of developing diseases such as cancer and obesity (Rybnikova et al. 2016; Lai et al. 2021). Although animal studies provide strong evidence that circadian disruption can lead to various diseases, including cancer, human studies are still less conclusive, largely due to confounding factors and methodological differences. As a result, the International Agency for Research on Cancer (IARC) has maintained its 2007 stance and classified night shift work as grade 2A evidence “probably carcinogenic to humans,” with strong mechanistic evidence in experimental models but limited evidence in humans (https://publications.iarc.fr/593). The ubiquitous presence of artificial light, both indoors and outdoors, especially during the natural periods of darkness, may have resulted in social factors playing a much more important role in circadian clock entrainment than in preindustrial times, namely by substantially controlling exposure to the key zeitgeber: light (Roenneberg and Merrow 2016). Notably, the activity patterns of humans (entrained phases or chronotypes) vary with individuals who are characterized as morning (lark), intermediate, or evening (owl) chronotypes. Overall, the analysis of self-assessment questionnaires (e.g., the Munich ChronoType Questionnaire [MCTQ]) shows that chronotypes are normally distributed in the general population, with ∼29% classified as morning, 41% classified as evening, and 30% classified as intermediate chronotypes (Roenneberg et al. 2019a). It is important to highlight that chronotype is highly influenced by the environmental light–dark cycle, including photoperiod (e.g., seasonal adaptation) (Allebrandt et al. 2014). In addition, studies indicate that people with an evening chronotype are more likely to suffer from social jet lag (e.g., a discrepancy in the rest periods between workdays and free days). Notably, social jet lag is associated with a higher risk for several health problems, including diabetes, cardiovascular disease, depression, and cancer (Roenneberg et al. 2012; Cruz et al. 2019; Giuntella and Mazzonna 2019; Min et al. 2023). As the extent of social jet lag depends on one's position in the respective time zone (i.e., same social times with variable solar time), this also plays a role in the ongoing debate about the negative effects of daylight saving time on health (Niu et al. 2024; Rishi et al. 2024). Due to the significant negative health effects of daylight saving time and its minimal impact on energy savings, most experts are calling for its abolition (Roenneberg et al. 2019b,c).
Another critical stressor for the circadian system is food intake, particularly highly processed foods, which are often high in calories and sugar and are constantly available. Several human studies have linked the consumption of ultraprocessed, high-fat, high-sugar foods to multiple diseases (Cordova et al. 2023; Lane et al. 2024). Although the constant availability of such foods is detrimental to health, restricting food intake to specific hours of the day, known as time-restricted eating, has shown significant effects in experimental models and modest benefits in humans. However, these studies often have limitations, including short-term observation and small sample sizes, that make it difficult to draw broad conclusions (Regmi and Heilbronn 2020; Chang et al. 2024; Ezpeleta et al. 2024). Although total energy expenditure (TEE) is similar between physically active hunter–gatherer populations and sedentary urban dwellers, the key difference lies in caloric intake. Modern lifestyles are characterized by increased consumption of high-calorie, processed foods, suggesting that increased caloric intake, rather than decreased physical activity, is the primary driver of the obesity epidemic (Pontzer et al. 2018). Not surprisingly, obesity and metabolic syndrome rates continue to rise worldwide, particularly alarming among children and adolescents (Ferreira et al. 2024).
Thus, the modern human lifestyle is a major stress factor for the circadian system (Fig. 1), as conflicting and misaligned zeitgebers can create an internal desynchrony (Boivin et al. 2022; Galinde et al. 2023) that negatively affects the physiological control of bodily functions and contributes to the development of various diseases (West and Bechtold 2015; Potter et al. 2016; Rijo-Ferreira and Takahashi 2019). It is important to note that the term “circadian disruption” is a broad description and can refer to one of three scenarios: loss or reduced amplitude of rhythms (e.g., flattening of serum cortisol or melatonin rhythms), internal desynchronization (i.e., when the clocks in different tissues show different phase relationships; e.g., often seen in shift workers), and circadian misalignment, when internal synchrony is maintained but misaligned with social demands and zeitgebers (Kramer et al. 2022).
The mammalian circadian network. Light is the most powerful zeitgeber, providing essential temporal information to the circadian system. Photoreceptors in the retina (specifically melanopsin in the retinal ganglion cells) detect light signals that are converted into electrical stimuli that travel along the retinal hypothalamic tract to the suprachiasmatic nucleus (SCN), the central clock in the brain. This process synchronizes the SCN with the geophysical time. Through a redundant network of temporal cues, such as body temperature, hormones, and autonomic innervation, a consistent internal time is established and maintained throughout the organism. Additional factors such as food intake and exercise can directly influence clocks in peripheral tissues and thus the temporal coordination of physiological processes. Stressors to the circadian system, such as light exposure at night, sleep disturbances, stress, sedentary lifestyles, shift work, and inappropriate dietary schedules, which are common in today's living conditions, can disrupt the fine-tuned regulation of the circadian system and negatively impact health and well-being. Image created with BioRender.
Physiological effects of circadian disruption
Sleep–wake cycle disturbances
The interaction between circadian rhythm (process C) and sleep homeostasis (process S) plays a critical role in the timing of sleep and wakefulness. The central clock controls process C to increase wakefulness during the day and sleep at night, whereas process S promotes sleep pressure with prolonged wakefulness requiring rest (Borbély et al. 2016). Light plays a key role in regulating melatonin production and adjusting the circadian rhythms, but this effect depends on the time of exposure. Exposure to light in the morning advances the circadian clock, whereas exposure in the evening delays it (Blume et al. 2019). Research by Roenneberg et al. (2003) showed that each hour spent outdoors advances sleep by almost 30 min (Roenneberg et al. 2003). Although such effects are not only dependent on light levels but also influenced by physical activity (Youngstedt et al. 2019), the timing of light exposure is always crucial. For example, light exposure during the day is associated with improved alertness and performance as well as reduced daytime sleepiness (Viola et al. 2008; Boubekri et al. 2014) and shows positive effects against sleep disorders (Faraut et al. 2020). Conversely, exposure to light at night, especially blue light, has a detrimental effect on sleep quality (Wams et al. 2017; Ishizawa et al. 2021), although reducing blue light from devices could improve it (Blume et al. 2019; Tam et al. 2021; Randjelović et al. 2023). For comprehensive reviews on the topic, the readers are referred to external reviews (Blume et al. 2019; Boivin et al. 2022; Wong and Bahmani 2022). The impact of shift work is of great relevance for health and health economics, as ∼20% of employees in industrial production, healthcare, the police, and many other sectors work night shifts, and sleep disorders are, not surprisingly, widespread among shift/night shift workers (Kerkhof 2018; Boersma et al. 2023).
Metabolic dysregulation
Previous research in mice has clearly shown that the absence of the circadian clock at the whole-body level disrupts several biological processes. These include, but are not limited to, premature aging, impaired energy metabolism, and increased susceptibility to metabolic diseases (Guan and Lazar 2021). Conversely, circadian clock knockouts in specific tissues only have less systemic impact, because the broader circadian network remains largely intact. In these cases, although the temporal coordination within the tissue may be affected, the systemic signals driven by the central clock may partially compensate for the absence of a local tissue clock (de Assis and Oster 2021; Guan and Lazar 2021). In humans, for example, shift work disrupts the precise timing and organization of biological processes, leading to internal desynchronization. This desynchronization occurs because activity and meals take place during normal rest periods, leading to a mismatch between expected and actual times for activity and rest (conflicting zeitgebers). Considering the multiple physiological alterations caused by circadian disruption (described in detail below), one might expect a higher incidence of metabolic dysfunction in shift workers (Fig. 2). In fact, a recent systematic meta-analysis showed that shift workers are more than twice as likely to develop metabolic syndrome as day workers (Sooriyaarachchi et al. 2022), and this association has been found in independent studies, albeit with different susceptibility ratios (Wang et al. 2014; Khosravipour et al. 2021; Shah et al. 2022). Considering the broad aspect of the metabolic dysregulation caused by circadian disruption, we have divided the consequences into subareas.
The consequences of circadian disruption. The temporal regulation of physiological processes is crucial for maintaining health. (Left panel) When the internal time of different organs is synchronized (i.e., in a stable phase relationship), biological processes exhibit precise circadian rhythms. This phase coherence between the central and peripheral clocks enables appropriate temporal control of various biological functions and thus promotes overall health. However, when this temporal control is disrupted, phase coherence between or within the central and peripheral clocks is compromised (shown by light arrows). This misalignment leads to a loss of temporal organization of various biological processes. (Right panel) There is growing evidence that such misalignment contributes to an increased incidence of several diseases, underscoring the importance of maintaining synchronized internal clocks for optimal health. Image created with BioRender.
Carbohydrate and lipid metabolism
Research on nurses working rotating shifts and who faced identical energy challenges found that night shift workers had elevated postprandial glucose levels and experienced a delayed glucose peak compared with day shift workers, indicating a lower responsiveness of pancreatic β cells to glucose (Sharma et al. 2017). Interestingly, avoiding meals during night shifts has been proposed to mitigate these metabolic effects (Grant et al. 2017). Compared with their daytime counterparts, rotating shift workers have higher average body mass index, blood pressure, triglycerides, and fasting insulin levels (Sookoian et al. 2007; Gowda et al. 2019). Consequently, shift work has been associated with higher hemoglobin A1c (HbA1c) levels, indicating impaired glucose metabolism in both rotating and nonrotating shift workers (Suwazono et al. 2009; Young et al. 2013; Manodpitipong et al. 2017). These findings are supported by a wealth of epidemiological studies that have found a link between shift work and an increased risk of developing type 2 diabetes (Shan et al. 2018; Vetter et al. 2018; Gao et al. 2020; Wang et al. 2023).
Shift workers were found to have higher total cholesterol and waist circumference (Gowda et al. 2019). A systematic review of nearly 200,000 shift workers found elevated triglycerides and lower cholesterol HDL levels, with permanent night shift workers at a higher risk (Dutheil et al. 2020). Not surprisingly, increased incidence and mortality of cardiovascular events have been associated with shift work (Torquati et al. 2018). It has been estimated that there is a 7.1% increase in the risk of a cardiovascular event for every additional 5 years of shift work, although the adverse effects of shift work do not appear to occur until after 5 years of shift work (Torquati et al. 2018). Therefore an important question remains as to how much a person can tolerate shift work. An independent study found an increase in cardiovascular risk and mortality of ∼5% after 5 years of shift work (Wang et al. 2018). Interestingly, the evening chronotype seems more susceptible to cardiovascular risk than the morning chronotype (Ritonja et al. 2019). A large population-based study from the United Kingdom showed that shift workers had a higher risk of incident and fatal cardiovascular disease compared with nonshift workers, which increased with shift duration. Importantly, this study identified modifiable risk factors, such as current smoking, short sleep duration, poor sleep quality, adiposity, higher glycated hemoglobin, and higher cystatin C that mediate the association with shift work and cardiovascular disease (Ho et al. 2022). Overall, shift workers are more likely to have metabolic syndrome, which is characterized by the presence of obesity, hypertension, dyslipidemia, and insulin resistance. In fact, a higher prevalence of metabolic syndrome correlates with short sleep duration (<6 h) compared with individuals with 7–8 h of sleep (Chaput et al. 2013; Smiley et al. 2019).
Gut microbiome
There is growing evidence that the microbiome is critical for healthy physiology when living in symbiosis with the host. However, an imbalance of the microbiome (i.e., dysbiosis) has been associated with several diseases (Hou et al. 2022). Interestingly, the gut microbiome also exhibits diurnal rhythms (Thaiss et al. 2014), which are influenced by a variety of factors, including the host's circadian clock and sex (Thaiss et al. 2014; Liang et al. 2015), diet (Leone et al. 2015), and timing of food intake (Thaiss et al. 2014; Ye et al. 2020). Further experimental evidence suggests that the microbiome contributes to the regulation of circadian rhythms in tissues, such as the gut (Mukherji et al. 2013; Wang et al. 2017) and liver (Montagner et al. 2016). It has been suggested that such control occurs via gut-derived metabolites (Ku et al. 2020) and short chain fatty acids (SCFAs) (Tahara et al. 2018). In addition, sleep disruption (Thaiss et al. 2014; Poroyko et al. 2016; Bowers et al. 2020) and obstructive sleep apnea (Moreno-Indias et al. 2015) affect the gut microbiome and can lead to dysbiosis. Circadian rhythms and dietary habits influence the gut microbiome in humans (Kaczmarek et al. 2017; Collado et al. 2018). Interestingly, germ-free mice transplanted with microbiota from jet-lagged humans (8–10 h phase advance) showed increased weight and blood glucose levels, which returned to normal after recovery from jet lag (Thaiss et al. 2014). There is an increased prevalence of gastrointestinal disease in shift workers, which has been linked to microbiome dysbiosis (Reynolds et al. 2017). The detrimental effects of shift work on the gut microbiome were found in a study with workers comparing day and night shift work (Mortaş et al. 2020). Although partial sleep deprivation under controlled laboratory conditions affected certain microbiome species without affecting β diversity (i.e., variation or dissimilarity in microbial composition between groups), further studies are needed to determine how shift work affects the gut microbiome (Benedict et al. 2016). For more information, the readers are referred to recent reviews (Teichman et al. 2020; Lopez et al. 2021).
Hormonal system
The circadian clock exerts a powerful control over the endocrine system. Several mammalian hormones exhibit clear circadian rhythms that are influenced by environmental factors, behavior, and the circadian clock itself (Gamble et al. 2014). Melatonin and cortisol are classic examples of hormones with clear circadian regulation that are affected by shift work. For example, reduced overall levels and delayed melatonin peaks have been found in shift workers compared with nonshift workers (Koshy et al. 2019; Razavi et al. 2019). Similarly, shift workers show higher urinary cortisol levels during daytime sleep and lower levels during nighttime sleep on off nights compared with day shift workers. In addition, shift workers have flatter cortisol rhythms (Manenschijn et al. 2011; Mirick et al. 2013; Hung et al. 2016). There is a large body of literature supporting the adverse effects of shift work on the endocrine system, focusing particularly on melatonin and cortisol (Boivin et al. 2022).
Chronic night shift nurses who have worked at least 1 year have higher insulin and leptin levels than day shift nurses, with no change in amplitude or phase. Importantly, chronic night shift work increases insulin levels that persist into nonworking days (Molzof et al. 2022). A similar increase in insulin and glucose levels has been reported in short-term shift work despite reduced leptin levels (Scheer et al. 2009). Conflicting reports of either unchanged (Bouillon-Minois et al. 2022) or decreased leptin concentrations in shift workers have also been reported (Crispim et al. 2011). A longitudinal study showed that night shift workers had higher leptin levels at noon than day shift workers. Early shift workers (6:00 a.m. to 2:00 p.m.) had higher leptin levels throughout the day (8:00 a.m. to 4:00 p.m.) compared with the day shift. Interestingly, acetylated ghrelin levels were lower only in early shift workers compared with day shift workers. Conversely, appetite quantification by questionnaire revealed lower appetite only in the early shift group (Crispim et al. 2011). A recent crossover study with a day and night shift protocol in chronic shift workers (two to five shifts per week for >1 year) found higher total acetylated ghrelin, with a more pronounced increase around wake time and increased appetite at breakfast in the night shift. However, no change in fasting and postprandial energy expenditure was observed (Qian et al. 2023).
A recent systemic analysis of 13 studies involving >110,000 subjects found lower vitamin D levels in shift workers, though with substantial variability between studies (Martelli et al. 2022). A recent study focusing on shift workers in the mining industry found that those with lower vitamin D levels had a twofold risk of developing hyperglycemia, characterized by elevated glucose and glycated hemoglobin levels. Notably, this association persisted even after adjustment for potential confounders, including sociodemographic, clinical, and anthropometric factors (de Almeida Santos et al. 2023).
Alteration in thyroid hormone levels has also been found in shift workers. For example, female nurses showed elevated thyroid-stimulating hormone (TSH) levels and a higher incidence of hypothyroidism in shift workers compared with nonshift workers (Moon et al. 2016). In contrast, a retrospective cohort study with 9 years of follow-up did not find elevated TSH levels in shift workers (Chen et al. 2021). However, a recent meta-analysis of three studies found a slight increase in the incidence of hypothyroidism in shift workers, though the data should be interpreted with caution due to the small sample size (Coppeta et al. 2020). Because serum TSH levels are rhythmic, controlling the time of TSH measurement (i.e., always in the morning or always in the afternoon) should be considered in the clinical setting (Russell et al. 2008).
With respect to sex hormones, night shift workers were shown to have higher levels of progestogens and androgens than day workers, even after controlling for other variables. These effects were observed only when men and women were analyzed together, whereas no significant changes were observed when analyzed by sex, a finding likely due to the small sample size in the subgroup analysis. Interestingly, night workers showed a delayed peak in androgen levels compared with day workers (Papantoniou et al. 2015). Corroborating evidence has shown delayed phases of androgens and progestogens in male shift workers compared with nonshift workers (Harding et al. 2022).
Overall, shift work disrupts the daily rhythms of several hormones (e.g., melatonin, cortisol, insulin, leptin, and ghrelin), leading to several adverse physiological outcomes, particularly in energy metabolism. Although much research has examined the short-term effects, fewer studies have examined the long-term consequences of shift work. Future research should prioritize understanding these long-term effects, investigate sex-specific hormonal responses, and develop strategies to mitigate the health risks for shift workers.
Immune system and inflammation
Immune system parameters, including the number of circulating hematopoietic cells and cytokines, are rhythmic and strongly influenced by the rest–activity phase. Blood levels of hematopoietic cells and mature leukocytes peak during the resting phase and decline during the activity phase. The sympathetic system controls this process by modulating the levels of CXCl12 and its receptor, CXCR4, in the bone marrow. On the other hand, the migration of hematopoietic cells and immune system cells to tissues occurs mainly during the active phase (Scheiermann et al. 2013). The nocturnal return of neutrophils to the bone marrow is thought to allow stromal cells to assess blood leukocyte levels, which may facilitate the next release of hematopoietic cells. In addition, the higher recruitment of leukocytes to peripheral tissues during the active phase may also serve to replenish the local immune cell reserves. Taken together, the circadian pattern of immune cell movement between the organism's comportments (blood and tissues) is suggested to be an evolutionary process to enhance the defensive capacity against infections, mostly during the active phase (Scheiermann et al. 2013; Labrecque and Cermakian 2015).
The consequences of rhythmicity in immune system parameters have been implicated in the therapeutic management of several chronic diseases with time-of-day-dependent incidence or symptoms. A classic example is the early morning stiffness in rheumatoid arthritis patients, which correlates with higher serum levels of inflammatory markers (Cutolo 2012). In addition, the incidence of stroke and myocardial infarction peaks in the early morning, which is thought to be due to increased sympathetic activity leading to an increase in blood pressure and blood coagulability (Suárez-Barrientos et al. 2011; Zeng et al. 2024). Another important consequence of the circadian rhythms in immune function is vaccination efficiency, as evidence suggests a time-of-day-dependent effect on vaccine response in humans (Long et al. 2016; Liu et al. 2021b; Otasowie et al. 2022).
There is a large body of literature on how circadian disruption affects immune system parameters. A cohort of 1351 men working in rotating shifts showed increased leukocyte counts in comparison with day workers, which was independent of age, smoking, and education (Sookoian et al. 2007). A study of 57 middle-aged nurses with a long history of shift work showed that working consecutive day and night shifts significantly decreased natural killer cell activity in the morning following the night shift (Nagai et al. 2011). Similarly, lymphocyte, T-cell, and CD8+ cell counts were higher in shift workers than in nonshift workers (Loef et al. 2019). A controlled study found that acute sleep deprivation led to increased cortisol levels, whereas chronic misalignment reduced 24 h levels of cortisol with associated changes in anti- and proinflammatory proteins (TNF-α, C-reactive protein, and IL-10 levels) (Wright et al. 2015). A cross-sectional study found increased immune cell counts in shift workers, with this effect being stronger in workers with a higher frequency of night shifts (Streng et al. 2022). For more information, the readers are referred to an external review (Faraut et al. 2013). Evidence of impaired immune system function can be inferred from susceptibility to SARS–CoV-2 infection, which is higher in shift workers (Fatima et al. 2021; Loef et al. 2022). Although another report did not find an increased incidence of SARS–CoV-2 infection, it did show an increased risk of moderate to severe infection and hospitalization among shift workers (Bjorvatn et al. 2023).
Thus, it is now beyond dispute that disruptions caused by shift work impair immune responses, increase inflammation, and increase the risk of infection. Future research should explore the long-term consequences of circadian disruption on the immune system. In addition, chronomodulated vaccine schedules represent a promising strategy to improve vaccine efficacy.
Circadian disruption and disease association
Cardiovascular diseases
Cardiovascular function exhibits rhythmic oscillations throughout the day. A classical example is blood pressure, which is lowest during sleep and rises sharply in the early morning (Degaute et al. 1991). Other cardiovascular features such as contractile function, cardiac output, and energy metabolism are also subject to circadian regulation (Güney et al. 1998; Young et al. 2001; Bray et al. 2008; Podobed et al. 2014). Experimental studies in mice in which clock genes (e.g., Bmal1 and Rev-erbα/β) were genetically deleted specifically in the heart revealed abolished cardiomyocyte clock rhythms, abolished day/night oscillations in sinus node beating rate (Young et al. 2014; D'Souza et al. 2021; Dierickx et al. 2022), and disturbed cardiac metabolism (e.g., glucose oxidation and triglyceride synthesis) (Tsai et al. 2010; McGinnis et al. 2017). Cardiovascular diseases such as myocardial infarction and stroke show an increased incidence in the early morning (Muller et al. 1985, 1989). Triggers for adverse cardiovascular events, including coagulation factors, inflammatory markers, shear stress, and autonomic tone, also fluctuate throughout the day (Young 2023). Single-nucleotide polymorphisms in PER2 and CRY2 have been associated with an increased risk of myocardial infarction (Škrlec et al. 2018). In addition, circadian disruption—either acute (e.g., switch to daylight saving time in the spring) or chronic (e.g., shift work)—has been associated with an increased risk of myocardial infarction (Janszky and Ljung 2008; Vyas et al. 2012; Wong et al. 2015; Morris et al. 2016). Interestingly, there is evidence that myocardial tolerance is lowest between 12:00 a.m. and 12:00 p.m. and particularly between 4:00 a.m. and 8:00 a.m. (De Luca et al. 2005; Suárez-Barrientos et al. 2011; Fournier et al. 2012).
Overall, circadian disruption has been associated with an increased risk of myocardial infarction and stroke, particularly in the early morning hours. Future research should focus on the molecular links between circadian disruption and cardiovascular disease, explore chronotherapeutic strategies to reduce these risks, and explore new methods to potentially identify workers at higher risk of circadian disruption for early intervention and prevention.
Metabolic disorders
Several lines of evidence show a strong association between circadian disruption caused by social jet lag (Parsons et al. 2015), late chronotype (Merikanto et al. 2013; Yu et al. 2015), and shift work (Eriksson et al. 2013; Liu et al. 2018; Wang et al. 2023) and poorer metabolic function (e.g., increased insulin resistance, diabetes, and weight gain/obesity). For example, controlled studies in humans that mimicked circadian disruption (e.g., shift work) found decreased muscle insulin sensitivity and higher serum glucose levels (Qian et al. 2018; Wefers et al. 2018), a common feature in shift workers (Shan et al. 2018; Vetter et al. 2018; Gao et al. 2020; Wang et al. 2023). Exposure to blue-wavelength-rich light, common in modern environments, has also been associated with circadian disruption and higher glucose levels (Cheung et al. 2016), providing another explanation for the phenotype exhibited by shift workers (Fleury et al. 2020). Polymorphisms in CLOCK have been associated with sleep duration and body weight in humans (Riestra et al. 2017) and possibly with the success of dietary interventions (Loria-Kohen et al. 2016). Importantly, there is robust evidence that shift workers are at higher risk of developing type 2 diabetes and obesity (Pan et al. 2011; Eriksson et al. 2013; Shan et al. 2018; Vetter et al. 2018; Khosravipour et al. 2021; Wang et al. 2023).
Neurodegenerative and psychiatric diseases
Accumulating evidence suggests a profound relationship between circadian disruption and the reduction of biological rhythms, such as sleep, which may serve as predictors for the development of Alzheimer's disease (AD) and Parkinson's disease (PD). Disruptions in circadian rhythms are associated with the worsening symptoms in AD and PD. Evidence suggests that alterations in rest/activity cycles and melatonin production may precede clinical motor symptoms in PD patients. In AD, sleep–wake cycle disturbances are commonly observed and are thought to contribute to the cognitive decline characteristic of the disease. Importantly, reduced amplitude of biological rhythms in sleep patterns, melatonin secretion, and body temperature are prevalent in AD and PD (Videnovic and Golombek 2017; Leng et al. 2019, 2020; Li et al. 2020; Fifel and Videnovic 2021; Zhang et al. 2022).
Circadian disruption is closely linked to mood disorders such as seasonal affective disorder (SAD) and depression. For example, SAD symptoms are exacerbated during fall and winter due to reduced daylight, and a misalignment of melatonin onset and bedtime that occurs in delayed sleep–wake phase disorder (DSWPD) is associated with increased depressive symptoms (Lewy et al. 2006; Chang et al. 2009; Walker et al. 2020). Major depressive disorder (MDD) is a highly burdensome global disease that negatively impacts quality of life and work productivity, with its prevalence increasing over the years (Proudman et al. 2021). Circadian disruption, as seen in shift workers, has been associated with an increased prevalence of MDD and anxiety (Torquati et al. 2019; Xu et al. 2023).
Patients with bipolar (BP) disorder experience extreme mood swings ranging from mania to depression, in addition to displaying changes in energy, sleep, appetite, and concentration throughout the day. Importantly, disruption of the sleep–wake cycle is a robust clinical feature of BP patients (Yan et al. 2023). BP patients show alterations in the rhythmicity of mood, energy, sleep, appetite, and concentration (Hensch et al. 2019), as well as multiple endocrine disruptions (Yan et al. 2023). Schizophrenia (SZ) is another disorder associated with disrupted circadian rhythms. Known for its strong genetic component, SZ manifests in various symptoms: positive such as delusions and hallucinations, negative such as anhedonia, and impaired cognitive abilities. Circadian disruption is a prevalent aspect of SZ and often exacerbates the severity of the disease (Ashton and Jagannath 2020; Boiko et al. 2024). Compared with healthy subjects, SZ patients lost the correlation between melatonin levels and sleep parameters (e.g., sleep latency, total sleep time, and sleep efficiency), which may contribute to circadian disruption and disrupted sleep patterns (Afonso et al. 2011).
Taken together, the intricate relationship between circadian disruption and various neurological and mood disorders confirms the critical role of biological rhythms in maintaining mental health. However, it remains to be determined whether circadian disruption is a cause or a consequence of the development of these disorders.
Cancer
A strong association between cancer onset and progression and circadian disruption has been described in experimental studies and, to some extent, in humans despite methodological caveats and a lack of concordance between the studies. Cancer progression has been conceptually categorized into 10 hallmarks, including sustained proliferative signaling, growth suppressor evasion, invasion and metastasis activation, replicative immortality, induced angiogenesis, resistance to cell death, deregulated cellular energetics, immune evasion, tumor-promoting inflammation, and genome instability and mutation (Hanahan and Weinberg 2011). Loss of circadian rhythms has not yet been considered as a possible hallmark, though it is a common factor in most hallmarks of cancer (Ortega-Campos et al. 2023). However, circadian rhythms are considered a hallmark of health (López-Otín and Kroemer 2021). Alterations in molecular clock gene rhythms have been identified in several human cancers, including melanoma (Lengyel et al. 2013; de Assis et al. 2018), liver (Liu et al. 2021a), breast (Fores-Martos et al. 2021), prostate (Kaakour et al. 2023), and others (Kelleher et al. 2014; Shilts et al. 2018; Ye et al. 2018; Wu et al. 2019; Huang et al. 2023).
An overwhelming body of experimental evidence shows that circadian disruption increases the likelihood of developing cancer in mouse studies. For example, chronic circadian disruption for 90 weeks led to fatty liver in all mice and hepatocarcinoma in ∼9% of wild-type mice, with higher rates in clock mutant mice (Kettner et al. 2016). In general, the strength of local circadian rhythms tends to decrease as cancer progresses, with complete loss of rhythms observed in some late stage cancers. This observation suggests a model of gradual disruption of circadian rhythms during cancer progression. However, the mechanistic relationship between circadian disruption and cancer remains largely elusive. The situation in humans is more complex, as conflicting studies suggest no clear effect of circadian disruption, making the issue less clear from an epidemiological standpoint. In this context, the IARC has recently maintained its classification of circadian disruption caused by shift work as probably carcinogenic to humans (grade 2A evidence) (https://publications.iarc.fr/593).
Circadian medicine
Over the past decades, there has been a growing awareness of how subjective/circadian time influences various physiological processes. Significant progress has been made in understanding the molecular mechanisms of the circadian clock. Although basic research has progressed considerably, the translation of these findings into clinical practice has been slower (Klerman et al. 2023). Only recently have the concepts of circadian medicine begun to be incorporated into clinical training, raising awareness among medical professionals of the critical role of timing in biological responses. This evolving landscape marks the emergence of a new field: circadian medicine.
We have identified three fronts on which circadian medicine is based (Kramer et al. 2022). The triad of circadian medicine are (1) detecting the clock, which is based on the development of new diagnostic tools that will allow tailored interventions according to the individual's circadian state; (2) targeting the clock, which aims to restore disrupted rhythms through various strategies; and (3) exploiting the clock, which is based on using knowledge of physiological rhythms to optimize treatment regimens (Fig. 3).
The triad of circadian medicine. Circadian medicine has many facets, all of which need to be advanced to sustain its development. They include assessing the status of a patient's circadian system to develop personalized treatment plans (“detecting the clock”), resynchronizing and strengthening the circadian system (“targeting the clock”), and treating and diagnosing at the right time of day (“exploiting the clock”). Each of these arms of circadian medicine involves a variety of approaches that need to be optimized, refined, and mechanistically underpinned for sustainable development of the emerging field of circadian medicine. Ideally, in the future, internal time will always be taken into account in the diagnosis, therapy, and prevention of disease.
Detecting the clock—new diagnostic approaches
The characterization of circadian rhythm parameters has advanced significantly in recent years with the development of novel methods. However, a recent meta-analysis of large circadian transcriptome data sets from mice revealed surprisingly low concordance between similar experiments. Interestingly, technical variation in sequencing strategy (i.e., library sequencing method) had a greater impact on variability than biological factors such as sex (Brooks et al. 2023). Evaluation of rhythm parameters in experimental studies is less complex than in humans because of the easier availability of tissue samples and a lower degree of interindividual variability (e.g., different chronotypes). Assessing internal time of humans is a step toward precision medicine and could be crucial for tailoring patient therapy (Kramer et al. 2022; Baum et al. 2023). It can be measured using questionnaires, actigraphy, and dim-light melatonin onset (DLMO) in blood or saliva (Reiter et al. 2020) or by analyzing clock gene expression in multiple samples taken at regular intervals from easily accessible peripheral tissues, such as hair follicles or oral mucosa (Dose et al. 2023; Liu et al. 2023).
In recent years, several approaches have been developed that require only a single human sample to determine internal time. For example, expression analysis of only a handful of genes from circulating monocyte cells shows high concordance with the phase determined from DLMO and thus represents a simple method for determining internal time in humans (Wittenbrink et al. 2018). Another challenge is the determination of internal time in retrospective samples collected without a clear temporal label. Methods such as CYCLOPS (Anafi et al. 2017) and CHIRAL (Talamanca et al. 2023) have been proposed to reconstruct circadian profiles. It should be noted, however, that the data sets used in these methods are from different individuals who died of different causes, raising questions about how this affected their internal rhythms. Nonetheless, the ability to temporally reconstruct samples across the day and obtain internal information from a single measurement is an exciting area in circadian medicine. Noninvasive longitudinal data collection using wearables can capture environmental data (e.g., light) and biological data (e.g., activity, heart rate, temperature, and blood oxygen levels) in uncontrolled environments (Shameer et al. 2017; Baum et al. 2023). Wrist actigraphy has been shown to predict internal time similarly to DLMO (Cheng et al. 2021). In addition, there are significant challenges associated with the collection, processing, standardization, and analysis of longitudinal human data that require further research for future clinical implementation (Baum et al. 2023).
Targeting the clock
“Targeting the clock” is based on using therapeutic approaches to strengthen or resynchronize circadian rhythms to re-establish temporal coordination of physiology and thereby influence disease outcomes. Classic modulators (zeitgebers) of the circadian system include light, food, hormones, and exercise. Restricting eating behavior to a limited time window (<10 h) and without caloric restriction has shown strong and promising effects in terms of metabolic health (e.g., glucose, triglycerides levels, and body weight loss) in mice, and there is some evidence for this also in humans, though limited by small cohort size and short duration (Adafer et al. 2020; Chang et al. 2024). Bright-light therapy has been used to treat a variety of sleep and psychiatric conditions, including circadian rhythm sleep disorders, seasonal affective disorder (SAD), and dementia (Shirani and St Louis 2009; Huang et al. 2024). Another strategy is physical exercise, which exerts a strong influence on physiology and has positive effects on health (Lewis et al. 2018; Gabriel and Zierath 2022; Martin et al. 2023). However, most of these approaches have yet to demonstrate conclusively that improved rhythms are the mediators of improved clinical outcomes. Direct modulation of the circadian system by small molecule modulators has also been identified, as several compounds with clinical potential have been identified (Chen et al. 2013; Sulli et al. 2018). Importantly, such strategies must take into account the natural aging process and be tailored on an aging population whose biological rhythms are gradually weakening (Sato et al. 2022; Olejniczak et al. 2023)
Exploiting the clock
Based on the body's natural rhythms, the idea of “exploiting the clock” is to use the existing rhythms to improve the treatment regimens for short-lived drugs, for example. Antihypertensive drugs taken in the evening such as angiotensin-converting enzymes (Hermida and Ayala 2009), angiotensin receptor blockers (Hermida et al. 2009), calcium channel blockers (Hermida et al. 2008b), or diuretics (Pickering et al. 1994; Hermida et al. 2008a) are more effective compared when taken in the morning. Evening administration of cholesterol-lowering drugs such as statins is also more beneficial than a morning administration, because endogenous cholesterol synthesis is highest in the evening. However, this only applies to short-lived statins (Awad et al. 2017). In healthy people, glucose levels rise before the active period, known as the “dawn phenomenon” (Schmidt et al. 1981). Glucose tolerance is rhythmic, being higher in the morning and lower in the evening as a result of reduced insulin sensitivity and pancreatic β-cell responsiveness (Lee et al. 1992). In contrast to the diurnal oscillation in nutrient consumption and energy uptake, continuous enteral feeding, which is commonly administered to critically ill patients, has been shown to exacerbate insulin resistance in these patients. In contrast, alternative approaches, such as intermittent feeding in critical care patients, have shown beneficial effects in mitigating this problem (Gonzalez et al. 2020). Therefore, consideration of rhythms in glucose metabolism may have important clinical implications in the management of diabetic and critically ill patients. Cortisol-associated syndromes also benefit from both a chrono-oriented diagnosis and therapy aiming to mimic the natural cortisol rhythm. Cushing's syndrome and adrenal insufficiency both need precise timing for cortisol measurement because of their opposing effects on cortisol levels. For example, in Cushing's syndrome, evening cortisol testing improves the diagnosis of abnormal hypercortisolemia (Viardot et al. 2005; Ceccato et al. 2013), whereas adrenal insufficiency is better detected in the morning when cortisol peaks (Montes-Villarreal et al. 2020; Kalaria et al. 2022). Surgical outcomes are also influenced by the time of day. For example, aortic valve replacement surgery appears to have fewer complications when performed in the afternoon than when performed in the morning (Montaigne et al. 2018). A recent study suggests that heart transplant recipients have higher survival rates when surgery is performed between midnight and noon than when it is performed between noon and midnight. The investigators speculate that this is due to increased markers of the immune system (e.g., antigen presentation) in the afternoon and evening, which correlate with increased rejection and mortality rates. However, the retrospective nature of this study and the inherent risk of bias from uncontrolled covariables significantly limit the strength of the study (Yim et al. 2023). Furthermore, some successful examples of chronotherapy for cancer have been reported. For example, chronotherapy with oxaliplatin, fluorouracil, and folinic acid in metastatic colorectal cancer reduced toxicity and improved treatment response compared with infusion at a constant rate (Lévi et al. 1997). However, a meta-analysis involving 842 patients with metastatic colorectal cancer found that three-drug chronotherapy significantly improved overall survival in males only, independent of known prognostic factors (Giacchetti et al. 2012). Patients with metastatic melanoma who received <20% of their immune checkpoint inhibitor infusions before 4:30 p.m. exhibited increased overall survival (Qian et al. 2021). Similarly, metastatic nonsmall cell lung cancer patients receiving immune checkpoint inhibitors in the morning showed improved progression-free survival and overall survival (Karaboué et al. 2022). An important example of how the concept of “exploiting the clock” can work is the evening intake of a modified release formulation of prednisone for rheumatoid arthritis patients. Delaying drug release to the very early morning hours effectively lowered interleukin-6 levels and improved clinical outcomes such as reduced joint stiffness (Buttgereit et al. 2008). For further examples, the readers are referred to a recent review (Cederroth et al. 2019).
Conclusion and future directions
It is our hope that readers lacking familiarity with circadian biology will recognize the crucial role that temporal regulation of physiological processes plays in maintaining health and well-being. For those already versed in the field, our aim is to provide a concise overview of the most recent advances. In this review, we have delineated the fundamental tenets of circadian biology, elucidating the manner by which our central circadian pacemaker receives, processes, and disseminates temporal information throughout the organism. Furthermore, we examined the impact of contemporary lifestyle factors, including dietary habits, work routines, light exposure, and exercise, on the circadian clock. Collectively, these factors can act as stressors on the circadian system. To contextualize these findings, we focused on shift workers, a population with markedly disrupted circadian rhythms. Experimental studies unequivocally demonstrate the detrimental effects of disrupted circadian rhythms due to various factors (e.g., light, food, and exercise). However, drawing robust conclusions from human studies is challenging due to sample heterogeneity and differing methodologies, as highlighted in the latest IARC evaluation of shift work.
Despite these limitations, we sought to elucidate the impact of shift work on physiological processes, including energy metabolism, the gut microbiome, hormones, and the immune system. Furthermore, we aimed to uncover the correlation between these changes and the increased risk of diseases such as metabolic disorders (e.g., obesity and diabetes), cardiovascular diseases (e.g., heart attack and stroke), neurodegenerative diseases (e.g., Alzheimer's and Parkinson's diseases), and cancer development. Although it is evident that disrupting circadian rhythms has profound consequences, there is a pressing need for greater standardization in studies to enable the drawing of more robust epidemiological conclusions. On a positive note, recent efforts are being made to incorporate circadian biology into the curriculum of health professionals, thus increasing awareness of the biological rhythms among the next generation of practitioners (Cedernaes et al. 2018; https://srbr.org/srbr-circadian-medicine-course-3).
Finally, we introduced the novel field of circadian medicine and delineated its key areas of focus: the detection, targeting, and exploitation of the clock. We undertook a comprehensive examination of their applications, current challenges, and potential benefits, which we believe should be the focus of basic and clinical research in the coming years. Our vision is that, in the future, internal time will be a central consideration in the diagnosis, therapy, and prevention of disease.
Competing interest statement
The authors declare no competing interests.
Acknowledgments
We apologize for omitting several landmark papers due to space restrictions. L.V.M.d.A. received a startup grant and funding from the Local Control of Thyroid Hormone Action (LOCOTACT) Consortium (project ID 424957847-TRR 296) and a Research Grant for Basic Science from the European Thyroid Association (ETA 2023). Work in A.K's laboratory is funded by the Deutsche Forschungsgemeinschaft (grants TRR186/P17, KR 1989/12-3, and KR 1989/13-1) and Horizon Europe (Shift2Health Consortium).
Footnotes
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Article published online ahead of print. Article and publication date are online at http://www.genesdev.org/cgi/doi/10.1101/gad.352180.124.
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Freely available online through the Genes & Development Open Access option.
This article, published in Genes & Development, is available under a Creative Commons License (Attribution-NonCommercial 4.0 International), as described at http://creativecommons.org/licenses/by-nc/4.0/.
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