For someone who reports being sleepy during the day, it is sometimes helpful to measure how sleepy he or she is. Also, after treatment of sleep problems, a medical professional may want to measure improvement in daytime sleepiness.

Sleepiness can be measured with a Multiple Sleep Latency Test (MSLT). Basically, the MSLT measures how fast someone falls asleep during the day. It must be done after an overnight sleep study (polysomnography) has documented adequate opportunity for sleep the night before. The test is composed of four to five "naps" that last 20 minutes each and are spaced two hours apart. The person is instructed to "try to fall asleep." The average time to fall asleep is calculated for all four or five tests. A normal time would be greater than ten minutes needed to fall asleep. Excessive sleepiness is defined as falling asleep in less than five minutes.

The Maintenance of Wakefulness Test (MWT) also measures daytime sleepiness. The person taking this test is instructed to "try to stay awake." This is repeated for four 40-minute sessions, two hours apart. Not falling asleep in all four tests is the strongest objective measure of the absence of daytime sleepiness.

Some businesses use these tests to ensure that their employees are not excessively sleepy while at work. Specifically, airline pilots and truck drivers who experience sleepiness need to have a test to ensure public safety and productivity at work. Unfortunately, there is no test that will guarantee that someone will not fall asleep at his or her job or while driving.

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