MSLT (Mean Sleep Latency Test)

Many people suffer from feeling sleepy throughout the day. They may express their symptoms as fatigue, groggy, exhausted, sluggish, low energy or worn out.  Being sleepy can make work more challenging and cuts down on family and fun time.  Therefore, it is important to pinpoint the degree of sleepiness and find out the cause.  The MSLT helps doctors rule out narcolepsy for example.

MSLT is a test of napping. Typically MSLT follows a prior night of sleep study (nocturnal polysomnogram) so you are likely be still hooked up to wires. You will be asked to take a total of 5 naps thorough the day. Each nap opportunity lasts approximately 20 to 35 minutes at 2 hour intervals (8 am, 10 am, 12 pm and so on). In between naps, you will be asked to get out of bed; you may sit in the chair or walk around, maybe watch TV or read. Once the naps resume, you will be instructed to “fall asleep”. The brain waves change as you drift from wakefulness to sleep and on to stage 1, 2, and perhaps 3 or REM sleep. This gives us information about how fast you fall asleep and whether you reach REM sleep (dreaming state) or deep sleep.

The test is performed in a room inside the sleep lab. Similar to the night time sleep study, the sleep technician will record brain waves, eye movements and chin muscle activity.  The technician will turn the lights off and ask you to fall asleep. Some sleep labs do drug testing to make take into account drugs that affect sleepiness or wakefulness. These might be stimulants or opiates.

The test is useful in identifying individuals who have sleep disorders such as narcolepsy, idiopathic hypersomnolence, or assess the degree of physiological sleepiness and the pattern of sleepiness.  If it takes you less than 8 minutes to fall asleep and your MSLT shows REM sleep on two naps, all in the setting of a fairly adequate and normal sleep the prior night (typically 6 hours), then this is highly suggestive of narcolepsy.

If your mean sleep latency is less than 5 minutes, it means you have severe daytime sleepiness; 5 to 8 minutes, moderate and between 8 and 10 minutes, mild sleepiness. These values do not apply during the night sleep study.