The Multiple Sleep Latency Test (MSLT) is a sleep disorder diagnostic tool. It is used to measure the time elapsed from the start of a daytime nap period to the first signs of sleep, called sleep latency. The test is based on the idea that the sleepier people are, the faster they will fall asleep.

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The MSLT is used extensively to test for narcolepsy in order to distinguish between physical tiredness and true excessive daytime sleepiness, or to assess whether treatments for breathing disorders are working. Its main purpose is to discover how readily a person will fall asleep in a conducive setting, how consistent or variable this is and the way they fall asleep in terms of Rapid Eye Movement sleep. This can be used to identify and differentiate between various sleep problems.

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The test consists of four or five 20-minute nap opportunities set two hours apart, often following an overnight sleep study. During the test, data such as the patient’s brain waves, electroencephalograph, muscle activity, and eye movements are monitored and recorded. The entire test normally takes about 7 hours during the course of a day.