Sleep latency — also called sleep onset latency — is the amount of time it takes you to go from being fully awake to sleeping. Sleep latency varies from person to person.

Your sleep latency and how quickly you reach rapid eye movement (REM) sleep can be indicators of the amount and quality of sleep you’re getting.

If you’re excessively sleepy during the time you should be awake and alert, your sleep latency could be a factor. Excessive daytime sleepiness can be a symptom of certain sleep disorders.

To diagnose a possible sleep disorder, your doctor may order a multiple sleep latency test (MSLT). This test measures how long it takes you to fall asleep during the day in a quiet environment.

Although the amount of sleep each of us gets varies, we generally spend about a third of our lives sleeping. Enough quality sleep is critical for a number of brain and other critical functions.

Sleep affects almost every type of tissue and system in your body, including the:

  • heart
  • brain
  • lungs

It also affects certain functions, such as:

  • metabolism
  • disease resistance
  • mood

Poor sleep quality or chronic lack of sleep can increase the risk of certain disorders, including:

Sleep latency is the amount of time it takes you to go from being fully awake to sleeping. It can play a role in sleep disorders.

Often referred to as a nap study, a multiple sleep latency test (MSLT) measures how long it takes you fall to asleep. It’s usually performed during the day in a quiet environment.

The MSLT test involves taking a total of five daytime naps scheduled two hours apart. You’ll be monitored to determine when you are in the following states:

  • awake
  • asleep
  • in REM sleep

If you fall asleep during your scheduled nap time, after 15 minutes of sleep you’ll be awakened. If you’re unable to sleep within 20 minutes, that nap will be cancelled.

Interpreting MSLT results

If you had no more than one nap in which you achieved REM sleep and your mean latency is below eight minutes, it’s possible you could have idiopathic hypersomnia. This condition results in excessive daytime sleepiness.

If you had no more than two naps in which you achieved REM sleep and your mean latency is below eight minutes, this may be a sign of narcolepsy. Symptoms of this disorder include falling asleep without warning, as well as excessive daytime sleepiness.

If your doctor recommends an MSLT, they’ll likely recommend that it immediately follow a polysomnography (PSG). A PSG is an overnight sleep study that monitors sleep cycles and sleep stages.

Results from this test might offer valuable diagnostic data on sleep issues that could impact your sleep latency, such as:

Enough quality sleep is essential for good mental and physical health. Your sleep latency — the amount of time it takes you to fall asleep — can be a good indicator of the quality of sleep you’re getting.