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The Multiple Sleep Latency Test (MSLT) is the gold standard in sleep tests, performed at most sleep clinics, and ordered by doctors in the diagnosis of many disorders.
The MSLT measures how fast a person falls asleep during the day, after a normal night’s sleep, in a quiet situation with little stimulation. Falling asleep quickly can be a sign of excessive sleepiness. The test also measures how soon the person enters REM sleep. Again, normal well rested people with no sleep disorders will not go into REM sleep during a short daytime nap. This period, called the sleep-onset REM period (SOREMPS), doesn’t occur for over an hour in a normal nighttime sleep, although some hypersomniacs have much shorter periods in daytime naps.
In the test, electrodes are connected to the facial muscles, scalp, and chest. It doesn’t hurt and you are either seated in a chair or lie on a sofa or in a bed.
The patient stops using stimulants and depressants several weeks before the test. Any drugs that influence the sleep architecture (antidepressants) should be stopped, too. On the day of the test, which generally happens in the morning, the patient does not exercise or become exposed to bright sunshine. The patient is monitored with polysomnography the previous night. For the MSLT to be valid, the patient must have slept for 6 hours minimum.
Depending on the protocol, the technicians will let the patient sleep a long time or wake him/her up after 15 or 20 minutes and go for a second time. Sometimes multiple naps are allowed, with a sleep latency and SOREMPS measured for each.
A doctor is usually not present for the MSLT. Technicians handle everything and give the results to the doctor for interpretation.
Sleep doctors respect the MSLT because the results are consistent from test to test, giving it the sheen of scientific respectability and increasing confidence.. However, it does not offer a definitive diagnosis for any disorder, partly because the medical profession has not established what is a normal sleep latency. Narcoleptics, for instance, typically fall asleep in less than 5 minutes, but this is not the same as saying people who fall asleep fast have narcolepsy necessarily.
In addition to narcolepsy, the MSLT is also valuable in diagnosis of periodic limb movement disorder, idiopathic hypersomnia, seizure disorders that are related to sleep, and apnea and other breathing problems. Doctors can distinguish between narcolepsy and hypersomnia from a MSLT.
It is also an expensive test and most often conducted at a specialized sleep center.
The MSLT is related to the maintenance of wakefulness test, but considered more authoritative and is used more widely. In addition to being used for patient diagnosis, these tests are used in scientific studies of many questions related to sleep, medicines, and wakefulness.
The result from an MSLT is a time, in minutes, to get to sleep. If the test is continues to look for REM, that time is also recorded as a result. Healthy people without sleep deprivation usually have a time to sleep exceeding 10 minutes during the day. An MSLT under 7 minutes is often taken as a sign of excessive daytime sleepiness.