A multiple sleep latency test (MSLT) measures how sleepy a person is during their waking hours. It can help doctors diagnose causes of excessive daytime sleepiness, such as narcolepsy.

An MSLT usually follows a polysomnography (PSG), which is an overnight test that monitors sleep cycles. The MSLT measures how long it takes a person to fall asleep during the day in a quiet environment.

In this article, we look at what an MSLT is, what doctors use it for, and what it involves. We also look at the results, whether the test is reliable, and what happens next.

A man in bed asleep with sensors attached to his skin as part of an MSLT.Share on Pinterest
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An MSLT, which people may also refer to as a daytime nap study, is a test that doctors use to evaluate a person’s ability to sleep during their waking hours in a quiet environment. It is based on the notion that sleep latency, meaning the time it takes to fall asleep, reflects how sleepy a person is overall.

The MSLT consists of four or five 20-minute naps during the day, which are all 2 hours apart. Researchers use the mean sleep latency from all of the naps to calculate an average result.

Doctors use MSLTs to diagnose certain sleep disorders, such as narcolepsy and idiopathic hypersomnia. Narcolepsy is a neurological condition that causes “sleep attacks” during the day, while idiopathic hypersomnia is excessive sleepiness with no perceivable cause.

A doctor may suggest this test for people who are sleepy during the day for no apparent reason. They might also recommend it for people who become sleepy in unusual situations, such as while working or driving.

Doctors may request that people prepare for the MSLT by:

  • keeping their sleep routine as normal as possible in the weeks leading up to the test
  • completing a sleep diary for 2 weeks before the sleep study to record the length and quality of their sleep
  • wearing a device called an actigraph on their wrist, which senses movement to help determine sleep patterns
  • letting doctors know which medications they take and checking whether they need to stop taking certain medications, such as stimulants for ADHD
  • avoiding foods and beverages that contain caffeine, such as coffee, tea, soda, and chocolate
  • bringing items they require for a good sleep to the sleep study, such as specific pillows or blankets
  • bringing items to entertain themselves between naps, such as a book to read

Doctors typically perform a PSG before an MSLT. This sleep study provides an evaluation of the person’s nighttime sleep. A person will usually need to stay overnight at a sleep center or in a hospital room.

For the test, a sleep technologist will use tape or glue to attach sensors to the person’s body to monitor them while they sleep. These sensors will monitor:

  • breathing
  • heart rate
  • brain waves
  • oxygen levels
  • leg movement
  • arm movement
  • eye movement

The person will still be able to move around, as the sensors attach to long wires that allow for mobility. A doctor or technologist will ask the person to move their limbs, move their eyes, and clench their teeth at the start of the study to ensure that the sensors are working.

A person can read, watch television, or otherwise entertain themselves before nighttime. When it is time to sleep, the technologist will turn off the lights and monitor the individual from a nearby room through a low light video camera. If the person needs to use the bathroom or a sensor comes loose, a technologist will assist.

Although a person may not be able to sleep as well during the study as they would at home, this should not affect the results, as a doctor does not need a person to sleep for the full 8 hours to make a diagnosis.

In the morning, the technologist will remove the sensors. They might ask the person to fill out a questionnaire about their sleep.

After the sleep study, a person can undergo an MSLT. This portion of the study typically lasts most of 1 day, during which a person will take four or five scheduled naps about 2 hours apart.

At the start, a sleep technologist will gently place sensors onto the person’s face and head. The sensors connect to a computer via wires.

The sensors show when the person is asleep or awake and transmit data to show when they are in rapid eye movement (REM) sleep.

The technologist will ask the individual to move their head and eyes and clench their teeth to test the sensors. They will then turn down the lights and observe the person via a camera from a nearby room.

The MSLT nap trial begins as soon as the lights go off. The MSLT will measure how long it takes a person to fall asleep and how long it takes them to reach REM sleep.

After 15 minutes of sleep, the technologist will wake the person. However, if the person could not fall asleep, the nap trial will end after 20 minutes. The individual will then have to remain awake for 2 hours, and the process will repeat three or four times until the MSLT is complete.

After the MSLT, a sleep technologist will review the data. They will look at when the person fell asleep during each nap and whether they entered REM sleep.

People with narcolepsy typically have two or more REM sleep stages during an MSLT. In contrast, people with idiopathic hypersomnia typically fall asleep easily but do not reach the REM stage during the test.

People with other conditions, such as sleep apnea, may also fall asleep easily during the trial. Doctors may be able to see indications of sleep apnea from the PSG the previous night.

After reviewing the data, a doctor will make a diagnosis based on the results.

MSLTs are generally reliable for diagnosing narcolepsy type 1. This form of narcolepsy includes cataplexy, which is a sudden loss of muscle tone in parts of the body.

However, a 2020 study notes that MSLTs are less reliable for diagnosing sleep disorders that do not cause cataplexy, such as narcolepsy type 2 and idiopathic hypersomnia.

Research suggests that, after repeated MSLTs, the test confirms a diagnosis in:

  • 91% of narcolepsy type 1 cases
  • 45% of narcolepsy type 2 cases
  • 49% of idiopathic hypersomnia cases

There are several reasons why the results can vary, and various factors can influence reliability. These include:

  • natural variations in sleep quality over time
  • coexisting conditions that may affect the results
  • medications or substances that a person has used but not disclosed
  • changes to a person’s physical activity level, diet, and lifestyle

After the sleep study, a person will have a follow-up appointment with a sleep specialist to discuss the results.

If the results from the first PSG and MSLT do not reveal anything but a person continues having symptoms that suggest narcolepsy or idiopathic hypersomnia, a doctor may decide to try these procedures again. Alternatively, they may try other tests to rule out other explanations.

If the results do indicate one of these conditions, the sleep specialist will explain the diagnosis they have made and work with the individual to put together a treatment plan.

What the treatment involves will vary depending on the diagnosis, but it may include:

  • medications to reduce excessive sleepiness in the daytime
  • changes to improve the person’s sleep, such as altering the sleep environment or making a bedtime routine
  • changes to the person’s diet or lifestyle

People with narcolepsy may also need to make changes to ensure their safety. For example, if they have sleep attacks or cataplexy while driving or operating machinery, this could put them and others in danger.

Learn more about treatments for narcolepsy and idiopathic hypersomnia.

An MSLT, or daytime nap test, helps doctors determine how long it takes a person to fall asleep during their waking hours. It also measures how quickly and how often they experience REM sleep. Doctors use this test to determine someone’s overall sleepiness during waking hours and to diagnose specific disorders.

A doctor may suggest a PSG and MSLT if they suspect narcolepsy or idiopathic hypersomnia. A PSG comprises the first part of the test, taking place at night, while the MSLT takes place the following day.

Before the test, a person may need to keep a sleep diary and avoid stimulants for 2 weeks. They may also need to wear a wrist device to monitor their movement during sleep.

MSLTs are often reliable in diagnosing narcolepsy type 1, but they may be less reliable for other types of disorder. People can ask their sleep specialist any questions about how the test works, its accuracy, and what the results may show.