Sleep Latency: It’s Importance in Diagnosis of Sleep Disorders

What is sleep latency? Simply put, it is the time taken to fall asleep. The unit of measure is minutes and it has an important role in the diagnosis of a number of sleep disorders including narcolepsy, hypersomnia and depression.

One is likely to hear the term often when doctor recommends a Multiple Sleep Latency Test, a test that essentially diagnoses daytime sleepiness. The test is done just after polysomnography. This is one of the most objective diagnostic tools that tests sleep onset rapid eye movement sleep periods (SOREMPS), useful for diagnosing narcolepsy.  According to the American Academy of Sleep Medicine (AASM), MSLT is considered to be the de facto standard for the objective measurement of sleepiness.

What is the test all about? 

Excessive daytime sleepiness and fatigue are common complaints (affecting 5% of the general population) with which patients visit sleep doctors. It is important to objectively evaluate and quantify these symptoms in order to diagnose the underlying disease condition as well as assess response to any ongoing treatment.

The test to assess sleep latency is based on the belief that one falls off to sleep faster with increasing rate of sleepiness. The test, also called the nap study, involves charting brain waves, heart rate and records eye and chin movements. It also records the speed with which one enters the REM stage of sleep and how often, during quiet situations during daytime.

The test efficiently eliminates factors that may interrupt sleep like uncomfortable temperatures, light, noise level, light intensity, etc. The results are affected by factors like anxiety, age, depression, drugs and medication, amount of sleep before the study, etc.

When is necessary to measure sleep latency? 

The test is considered to be a standard diagnostic tool for patients who are suspected to be suffering from narcolepsy.  The test is also used for testing idiopathic hypersomnia. Well-rested adults have a mean MSLT of 15 minutes. A mean MSLT of less than 5 minutes indicates severe hypersomnia.

How is the procedure conducted? 

The nap study typically consists of five opportunities to nap at 2 hour intervals. This means there are 2-hour gaps after each nap session. The naps are conducted in cool, dark, comfortable and sleep-friendly environments. The first nap begins about 1.5 to 3 hours after waking up from nighttime sleep.

Sensors designed to send electrical signals to a computer are placed on the head, face, and chin. They show when the patient has fallen asleep and awake. The brain-wave and eye-movement detectors display the time when the patient has entered REM sleep. During this stage of sleep, there is eye movement and the brain is active.

Prior to taking the nap, the patient visits the bathroom if necessary and should be in bed at least 5 minutes before starting of the test.

For each nap the patient is asked to lie down in a comfortable position with eyes closed and the lights are switched off before start of the test. For ensuring accuracy of the test results, a total sleep time of at least six hours during the overnight sleep study is recommended.

How do sleep latency scores signify degree of sleepiness

It is accepted that a mean sleep latency of < 5 min indicates a pathologic level daytime sleepiness. 

MSLT Scores Minutes

Sleepiness

0–5

Severe

5–10

Troublesome

10–15

Manageable

15–20

Excellent

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