What Is a Multiple Sleep Latency Test (MSLT)?

Medically Reviewed by Brunilda Nazario, MD on July 17, 2023
Written by Kendall K. Morgan
3 min read

If you are unusually sleepy during the day when you should be awake, your doctor might recommend you have a multiple sleep latency test (MSLT). Sometimes, it’s also called a daytime nap study.

While another sleep test called polysomnography measures how you sleep at night, MSLT looks at how fast you fall asleep during the day. “Sleep latency" means the time it takes you to fall asleep. The goal is to find out whether you are unusually sleepy during the day by seeing what happens if you lie down for several naps over the course of a single day.

Polysomnography is usually used to diagnose narcolepsy. or obstructive sleep apnea, in which breathing starts and stops during sleep. But this use of the test isn’t as common. Sometimes a doctor will just ask about how sleepy you feel instead.

Lots of things can affect your MSLT results. These include:

Because MSLT results depend on all these things, your doctor may ask you to take certain steps before your MSLT. These include:

  • Do your best to sleep normally leading up to your test.
  • Keep track of how you are sleeping and any things that might be causing sleep trouble and making you sleepy during the day.
  • Talk to your doctor about any drugs or medicines you take and how they may affect your sleep or sleepiness. Your doctor might ask you to stop using some medicines or other substances before your test.
  • You’ll likely have a sleep study the night before the MSLT. You should sleep for at least 6 hours the night before your test.

 

You should expect your MSLT to take most of your day. During the test, you’ll try to take five naps at set times with 2-hour breaks in between. Sometimes, only four naps is enough.

You will have sensors put on your head, face, and chin. These sensors will track whether you are awake or asleep. They’ll also tell if you are in rapid eye movement (REM) sleep. People with narcolepsy not only fall asleep fast, but they also enter REM sleep more quickly than normal.

The person doing your test will make sure the sensors are working. They’ll also use a low-light video camera to watch and make sure you are OK throughout the test.

Each nap will follow these steps:

  • With the lights turned off, you’ll lie quietly to try to go to sleep.
  • If you fall asleep, they’ll wake you up after a 15-minute nap.
  • If you don’t fall asleep in 20 minutes, they’ll end the nap trial.
  • After the trial, you’ll wait 2 hours for the next one until you’ve finished all four or five nap trials.

 

It can take a couple of weeks to get your MSLT results. Experts will look at how fast you fell asleep and whether you entered REM sleep. If you have narcolepsy, it’s common to fall asleep fast and enter REM sleep more than once over the course of the MSLT. People with narcolepsy who take the MSLT usually fall asleep in less than 8 minutes, on average.

If you are sleepy or sleeping a lot for some other reason, you might fall asleep fast without entering REM sleep quickly. Other conditions such as sleep apnea also can make you fall asleep quickly during the MSLT.

Keep in mind that it’s possible to feel very sleepy during the day and still not show that you have narcolepsy or another sleep disorder based on your MSLT results. Your sleep doctor can help you figure out what your results mean and what you should do next.