Most sleep studies are performed overnight so your doctor can monitor your sleep patterns and behavior. But you may also benefit from another test that assesses just how sleepy you are during daytime hours.
The Multiple Sleep Latency Test (MSLT) looks at how overtired you are during the day. It determines the severity of hypersomnolence, which is an overwhelming need to fall sleep. Your doctor will schedule an MSLT first thing in the morning on the day immediately following an overnight sleep study. Before you start the test, electrodes are placed on your head and elsewhere on your body, and other measurement devices may be attached somewhere on your face or on a finger. For the most part, the same measurements that were taken during the overnight study will be taken during the daytime study.
During the MSLT, you will take four or five 20 to 30-minute naps, every two hours, throughout the day. You will be asked to lie quietly and comfortably, with your eyes closed and try to fall asleep. Between naps, you will get out of bed. You can eat breakfast, lunch, and snacks, just as you do on a normal day. You can read, watch television, or do any quiet activity you bring with you to the test, but phone calls and visitors are not usually permitted because they can interfere with the results of the test.
The electrodes attached to your body measure the changes that occur in your brain as you fall asleep. Your degree of hypersomnolence is measured by sleep latency, or the amount of time it takes for you to fall asleep once the lights are turned off. It usually takes more than 10 minutes for normal sleepers to fall asleep. The MSLT also confirms the existence of a rapid eye movement (REM) stage of sleep while napping and measures how quickly you reach REM sleep. Normally, REM sleep does not occur during a nap.
The MSLT helps confirm a diagnosis of narcolepsy, a condition in which a person suddenly loses muscle control and falls asleep, or idiopathic hypersomnia, which is excess or inappropriate daytime sleepiness without an apparent cause. Unlike normal sleepers, most people with narcolepsy or idiopathic hypersomnia fall asleep within five minutes and experience REM sleep while they nap. These conditions can be difficult to diagnose because routine overnight sleep studies often show normal sleep patterns and behavior, even in someone who is chronically tired or falling asleep at inappropriate times during the day. The MSLT, in combination with an overnight sleep study, gives the doctor a clear picture of your overall sleep patterns and behavior.
Sources:
Littner, MR, et al; "Practice Parameters for Clinical Use of the Multiple Sleep Latency Test and the Maintenance of Wakefulness Test"; SLEEP 28(1) 2005 Web. 24 Jun 2011.
http://psychiatry.duke.edu/wysiwyg/downloads/2005_Practice_parameters__clinical_use_of_MSLT.pdf
MedlinePlus: Idiopathic Hypersomnia. 22 June 2011. Web. 24 June 2011.
http://www.nlm.nih.gov/medlineplus/ency/article/000803.htm
