Multiple Sleep Latency Testing (MSLT)

Multiple sleep latency testing (MSLT) is an objective measurement of how long it takes you to fall asleep. It is often called a nap study. The set-up is similar to that used for a polysomnogram.

Man in a sleepy study
Jeff T. Green / Stringer / Getty Images 

How MSLT Is Conducted

The testing involves a visit to a sleep center, which may consist of specially designated rooms in a hospital, sleep laboratory, or even a specially equipped hotel room. Typically, after spending a night undergoing a standard polysomnogram, the next day is devoted to conducting MSLT.

After waking up, you will have scheduled naps at two-hour intervals throughout the day. These consist of 20-minute periods where you are put to bed and allowed to lie there with the goal of falling asleep. The sleep technician monitors you to see when you fall asleep and, in particular, when REM sleep begins. Various physiological parameters are monitored while you sleep, including an EEG, EKG, breathing, oxygen levels, muscle tone, and eye and extremity movements. There is also a video and audio recording that provides a record of the night’s sleep.

After 20 minutes, you are awakened or told that the time for a nap has ended. This process is repeated every two hours. Typically these scheduled naps will occur five times.

What MSLT Measures

One of the primary measures of MSLT is sleep latency, or how long it takes you to fall asleep. The premise is that people with excessive daytime sleepiness will fall asleep more quickly, and thus this testing should reveal this phenomenon. There is a wide range of normal sleep latencies, and a short one may not necessarily suggest a sleep disorder. As with any sleep testing, it is important to undertake the tests for the right reason and any results must be understood in the proper clinical context.

Excessive daytime sleepiness may be present in numerous disorders, such as idiopathic hypersomnia (excessive sleepiness without a cause) and narcolepsy. In particular, the early onset of REM in these sleep periods may suggest narcolepsy. If there are no rapid eye movements soon after falling asleep, it may instead suggest idiopathic hypersomnia.

Although excessive daytime sleepiness may also occur in sleep apnea, MSLT is not routinely conducted in those with this condition.

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