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Jul 09, 2021
10 min read

Sleep latency: what is it, testing, and results

Does this sound familiar? You turn off the lights at bedtime, lay in bed, but you can’t go to sleep. Or maybe you nod off during a meeting or can barely get through a conversation without falling asleep. Excessive daytime sleepiness or the inability to fall asleep at night are two ends of the sleep latency spectrum.

Disclaimer

If you have any medical questions or concerns, please talk to your healthcare provider. The articles on Health Guide are underpinned by peer-reviewed research and information drawn from medical societies and governmental agencies. However, they are not a substitute for professional medical advice, diagnosis, or treatment.

Does this sound familiar? You turn off the lights at bedtime, lay in bed, but you can’t go to sleep. Or maybe you nod off during a meeting or can barely get through a conversation without falling asleep. Excessive daytime sleepiness or the inability to fall asleep at night are two ends of the sleep latency spectrum.

Tracking sleep latency, the time it takes to fall asleep, also known as sleep onset latency (SOL), is a way to help diagnose and discuss sleep problems.  

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What is sleep latency?

Sleep latency is the amount of time it takes to go from wakefulness to sleep. It’s recorded in a clinical setting during the day, usually only when a healthcare provider needs to determine a diagnosis of narcolepsy or idiopathic hypersomnia. These are potentially dangerous sleep disorders marked by excessive daytime sleepiness (Arand, 2019; Khan, 2015; Berkowski, 2016).

Studies show those with narcolepsy, hypersomnia, or sleep deprivation are more likely to be in a motor vehicle accident than the general population (Tzeng, 2019, Pizza, 2015; Gottlieb, 2018). 

You can also measure sleep latency informally at night as part of an insomnia diagnosis. If you’re suffering from sleep deprivation, your healthcare provider may ask you to keep track of how long it takes to fall asleep each night and the amount of sleep you get. It’s not as scientific or thorough as the clinical sleep latency testing to diagnose narcolepsy, but it is included in a sleep history and is part of a diagnosis (Bollu, 2019). 

Overall, sleep latency is just one measurement used to better understand sleep patterns and potential sleep disorders. It’s combined with other information and test results to help diagnose sleep problems. 

How well do you sleep?

If you’re one of the many who try to catch up on sleep on weekends, it may be unavoidable if you have a hectic schedule. But it’s not ideal. Sleep medicine experts say a good night’s sleep each night helps keep sleep cycles regular, which is better for your health, metabolism, and mental outlook. (Depner, 2019; Le Bras, 2019; Walker, 2020; Owen, 2020) That said, many people suffer from sleep problems. 

Daytime sleepiness or the inability to sleep at night are common and may be temporary. For example, you may be nervous about an upcoming event, and you aren’t sleeping well. Worrying has been shown to make it harder to fall asleep—extending sleep latency (Clancy, 2020). A lack of nighttime sleep can then make you sleepy the next day.

If sleep problems are affecting your life, contact your healthcare provider. Creating a detailed sleep history that includes sleep latency is often the first step in figuring out the problem. Lifestyle factors or underlying medical conditions could be to blame. Here are a few topics your healthcare provider may discuss:

Sleep diaries and questionnaires

Healthcare providers use self-reported questionnaires and sleep diaries to sort out the extent and causes of sleep problems. Questionnaires such as the Epworth Sleepiness Scale, the Pittsburgh Sleep Quality Index, and the Insomnia Severity Index are used to start an evaluation (Bollu, 2019; Ibanez, 2018).

Sleep diaries go more in-depth and track sleep latency, total sleep time, how often you wake up, along with daytime napping, caffeine, alcohol, medications, and activities that may affect sleep (Bollu, 2019). Sleep medicine experts created a consensus sleep diary that offers an example of a comprehensive diary format (Carney, 2012). Tracking sleep patterns for at least two weeks is often required before clinical sleep tests are conducted. 

Sleep cycles

Not all sleep is equal. We need to rotate through these four (sometimes classified as five) stages of sleep at the right intervals each night (Patel, 2021):  

  • N1 (stage 1): The lightest stage of sleep; typically lasts from 1–5 minutes
  • N2 (stage 2): Deeper than stage 1; heart rate and body temperature drop; lasts about 25 minutes at first and gets longer each cycle to reach 50% of sleep; linked to memory and learning (Purcell, 2017)
  • N3 (stage 3): Deepest stage; body repairs tissues and boosts immunity during deep sleep (Patel, 2021)
  • REM (rapid eye movement) sleep: Marked by dreams and muscle impairment; typically begins about 90-minutes after sleep; length of REM periods increase from 10 minutes to an hour as sleep cycles continue; linked to cognitive health (Song, 2015; Scullin, 2015

If sleep latency is extended at night, and you lay awake, it pushes back sleep cycles, and a morning alarm is likely to interrupt deep or REM sleep. Studies have shown that interrupting sleep cycles and sleep loss, particularly deep and REM sleep, negatively impacts sleep quality, overall health, longevity, learning, and memory. (Grandner, 2017; Mazzotti, 2014; Goel, 2009; Cairney, 2018)

Circadian rhythm

The circadian rhythm is the 24-hour internal clock that regulates cycles of wakefulness and sleepiness based on light changes. Disrupting your circadian rhythm can negatively affect many aspects of your health (Reddy, 2021). 

Sleep hygiene 

You may be asked to review your lifestyle and improve your sleep hygiene (Shrivastava, 2014). This involves maintaining regular sleep and wake time schedules, avoiding evening caffeine, smoking, and alcohol, adopting healthy diet and exercise regimens, and avoiding daytime naps (Kaur, 2020).

Watching television and using a smartphone while in bed are associated with longer sleep latency and are not advised (Exelmans, 2018). The goal is to maximize sleep efficiency each night, which lets you wake up feeling restored without wasting time. 

Underlying medical conditions

Discuss any potential medical or mental issues that could be causing sleep disturbance. Thyroid, liver, and kidney function tests and blood and iron assessments are initial tests used to evaluate underlying medical conditions linked to insomnia (Kaur, 2020).

An overnight sleep study called a polysomnogram (PSG) can also help to rule out or diagnose underlying medical conditions that lead to insufficient sleep, such as obstructive sleep apnea (OSA) or periodic limb movement disorder (Rundo, 2019). These both result in continual awakenings. A person with OSA stops breathing at different times. This interrupts sleep and leaves them feeling exhausted. A CPAP machine (continuous positive airway pressure) is typically used to restore seamless nighttime breathing (Calik, 2016). 

Sleep latency testing

Clinical sleep latency testing is typically used to diagnose cases of excessive daytime sleepiness, including narcolepsy. The Multiple Sleep Latency Test and the Maintenance of Wakefulness Test are variations of the same test.

Multiple Sleep Latency Test

The Multiple Sleep Latency Test (MSLT), sometimes called a nap study, is the gold standard for evaluating excessive daytime sleepiness (Arand, 2019). The test, often conducted at a sleep center, involves laying down in a quiet environment with the lights off. The patient then takes 4-5 20-minute naps at 2-hour intervals throughout the day (Goddard, 2021). 

The MSLT test involves an electroencephalogram (EEG) and an electrocardiogram (EKG or ECG) to measure brain waves, heart rate, eye movements, and muscle activity (Rayi, 2021; Satar, 2021). The test identifies when someone falls asleep and if rapid eye movement (REM) sleep occurs. The average person typically achieves REM sleep after at least 90-minutes of sleep (Patel, 2021). The early arrival of REM sleep, tracked by measuring the sleep-onset REM period (SOREMP), is a feature of some sleep disorders (Slowik, 2020).

The MSLT test is scheduled the day after polysomnography if the participant achieved at least six hours of sleep during that test (Arand, 2007). The combined tests allow healthcare providers a better window into what may be affecting sleep and sleep quality.

If you’re scheduled for an MSLT test, your healthcare provider will review instructions. You may be asked to bring a sleep diary, temporarily discontinue some medications, and avoid caffeine and alcohol leading up to the test. A urinary drug screening test may be required on the day of the test (AASM, 2020-a; Anniss, 2016). 

What do MSLT test results show?

A mean sleep latency (SOL) between 10–20 minutes is considered normal for an adult. A mean SOL equal to or under 8 minutes with 2 or more SOREMPs can indicate narcolepsy (Muza, 2016). The hallmark of narcolepsy is decreased REM sleep that starts within 15 minutes of sleep onset and frequent nighttime awakening, resulting in excessive daytime sleepiness. An idiopathic hypersomnia diagnosis involves a similar SOL but with no more than one REM period (Trotti, 2017). Healthcare providers use this test as part of a diagnosis. An MSLT test alone does not confirm or rule out a diagnosis (Dietmann, 2021).

Maintenance of Wakefulness Test 

The Maintenance of Wakefulness Test (MWT) is a variation of the MSLT but is designed to test someone’s ability to stay awake under conditions that may elicit sleep. Unlike the MSLT test, you are not lying down. Instead, the participant is seated in a quiet, dimly lit room without stimuli. They are asked to look forward and try to stay awake without actions like singing, looking at lights, or other physical activity that might keep them awake. 

There are four potential naps (or wakeful periods) with two-hour intervals. MWT test naps can last up to 40 minutes (Tanaka, 2015). A mean SOL under eight minutes is considered abnormal by the American Academy of Sleep Medicine (AASM, 2020-b). Research is underway to see if MWT results can predict an increased risk of motor vehicle accidents (Philip, 2021).

References

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