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What is a Multiple Sleep Latency Test (MSLT)?

You recently learned about the sleep disorder, narcolepsy, here at Sound Sleep Health ( Narcolepsy: Signs, Symptoms, and Diagnosis ). Critical to arriving at a diagnosis is the use of a specific kind of daytime sleep test known as the MSLT, or the multiple sleep latency test, to confirm a suspicion of narcolepsy, as well as to differentiate it from the presence of a similar condition, idiopathic hypersomnia.

How is narcolepsy diagnosed?

As with many other sleep disorder diagnoses, a diagnosis of narcolepsy occurs only after the following have been considered:

Patient history.Certain markers of narcolepsy, such as age and family history, are noted. Also, other preexisting or yet-unidentified health conditions which might mimic narcolepsy are also taken into consideration.

Symptoms.The four key symptoms of narcolepsy are discussed:

Sleep diary data.Patients will be asked to record sleep patterns for a couple of weeks prior to any testing to give the physician insight into sleep hygiene, average sleep amounts, and other details which can help with testing conclusions.

Testing.The narcolepsy testing protocol usually includes the following:

  • Overnight polysomnogram. This overnight sleep study captures important data about rapid-eye movement, sleep efficiency, and how quickly a patient falls asleep. It also measures for signs of other potential sleep disorders that could be held (at least partially) to account for the symptoms connected to narcolepsy, such as movement disorders of sleep or sleep breathing disorders like sleep apnea.
  • Multiple Sleep Latency Test (MSLT).The MSLT is a daytime sleep test for narcolepsy which involves recording brainwave activity in patients during the day as they nap. This information is critical to differentiating a diagnosis between narcolepsy and idiopathic hypersomnia (more below).

The sleep study with MSLT usually occurs over the course of a night and into the following day.

Testing for narcolepsy may require that patients on certain kinds of medications cease taking them prior to the MSLT to ensure results of the tests aren’t distorted by the side effects of these drugs.

What is an MSLT?

The patient undergoing an MSLT is asked to spend the day in the sleep center laboratory.

Meals during the MSLT include a light breakfast an hour before the first nap trial, and lunch provided immediately after the second nap.

The patient is asked to take as many as 5 naps at regular 2-hour intervals over the course of the day.

They will wear some of the same sensors they wore during the overnight test, such as the ones already applied to the head, face, and chin, but others are no longer necessary and will be removed.

At the beginning of each nap test, the sleep technologist will enter and darken the room and ask the patient climb into bed to attempt to fall asleep. After 15 minutes have passed, the technologist will check remotely (by way of the sensors) to see if the patient has fallen asleep.

  • If the patient hasn’t fallen asleep in 15 minutes, they will be given another 5 minutes to do so, and if they remain awake at the 20-minute mark, their test will conclude.

  • If the patient has, in fact, fallen asleep, the technologist will awaken the patient at 15 minutes to conclude the study.

In both cases, once the sleep trial concludes, the technologist will reintroduce light into the test room and ask the patient to stay busy (reading a book, watching TV, computer work, knitting, etc.) until the next nap interval arrives, roughly 2 hours afterward.

If the patient does not fall asleep in the first 4nap tests, the sleep technologist may opt to conclude the MSLT, allowing the patient to go home early. However, if the patient has fallen asleep at least once during the first first trials, they will be asked to stay for a final fifth test.

The naps take place so that the sleep technologist can measure for the following:

  • Wake time during naps:Did they stay awake through the entire nap?
  • Sleep time during naps:Did they fall asleep during the nap? How much did they sleep?
  • Sleep stages during naps:If they fell asleep during the nap, what sleep stages were recorded during the test?
  • Time to sleep onset:If they fell asleep during the nap, how long did it take for them to do so?

Ultimately, the MSLT is looking for two specific details:sleep latency and sleep onset REM periods.

Sleep latency

This refers to the period of time it takes from the moment the test begins to the moment the patient falls asleep. Sleep, in this case, is defined as 90 continuous seconds of light (stage 1) sleep or 30 seconds of any other kind of sleep.

Sleep onset REM periods

This refers to the appearance of REM-phase sleep following the actual onset of sleep. In normal sleep architecture, a patient may take as much as 90 minutes to achieve stage REM sleep.

However, in narcoleptics, a quick movement directly into REM sleep following sleep onset is marked as a sleep onset REM period (or SOREMP).

A person who has an overnight sleep study with a followup MSLT the next day can be diagnosed with narcolepsy if they experience 2 or more SOREMPs either during the MSLT itself or in combination with the previous nights sleep study.

What does this have to do with Idiopathic Hypersomnia?

Idiopathic Hypersomniais a diagnosis of extreme sleepiness which cannot be explained otherwise (due to poor sleep hygiene, an underlying sleep disorder, or other medical condition),and does not qualify as narcolepsy due to an absence of SOREMPs and naps.

As with any idiopathic condition (meaning its origin is unknown), a patient with idiopathic hypersomnia may not necessarily receive the same treatment as someone with narcolepsy because enough isn’t known about their condition to proceed confidently with a specific therapy.

This is why the MSLT is such an important diagnostic test for differentiating narcolepsy from idiopathic hypersomnia, because a positive diagnosis for narcolepsy allows the sleep physician to act on specific treatment protocols.

Narcolepsy is not a common sleep disorder, but it’s not rare either, and researchers think it may be under diagnosed. Having patients suspected of having narcolepsy participate in an MSLT is currently the American Academy of Sleep Medicine protocol for sleep clinics. A night of sleep and a day of naps may be all it takes to uncover those suffering from this life-altering sleep disorder and find pathways for treatment.

People who feel sleepy all the time and cannot easily explain why are encouraged to bring up this concern with their physicians. It’s common to have an underlying sleep disorder and not know it. Since all sleep disorders are treatable, there’s no reason not to consult about sleep problems so they can be identified and treated.


American Academy of Sleep Medicine
Healthy Sleep (
National Center for Biotechnology Information
Sleep Review Magazine

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