MSLT vs MWT: The Two Daytime Sleep Tests Explained

The MSLT and MWT are daytime sleep tests done in a sleep lab. The Multiple Sleep Latency Test (MSLT) measures how fast you fall asleep during planned naps, and it is the main test used to help diagnose narcolepsy. The Maintenance of Wakefulness Test (MWT) measures how well you can stay awake, and it is mostly used to check alertness for safety, such as driving.

MSLT vs MWT: what each test asks you to do

Both tests use the same sensors as an overnight sleep study: small stickers on the head and face that track brain waves, eye movements and muscle tone. The big difference is the instruction you get.

• MSLT: "Try to fall asleep" — you lie down in a dark, quiet room several times during the day. The test measures how quickly you fall asleep and whether you go into REM (dream) sleep early.
• MWT: "Try to stay awake" — you sit up in a dim, quiet room and try not to fall asleep. The test measures how long you can stay awake.

A sleep doctor usually orders these tests after an office visit. To see how they fit with other sleep tests, read how sleep doctors diagnose sleep disorders.

The night before: an overnight sleep study

An MSLT is done the morning after an overnight in-lab sleep study, called polysomnography (PSG).^[1] The overnight study has two jobs:

• Check for other sleep problems — such as sleep apnea, which can also cause daytime sleepiness. You can learn more in how polysomnography works.
• Make sure you slept enough — AASM guidance recommends at least 6 hours of sleep on the overnight study. Too little sleep can make anyone fall asleep fast the next day.^[1]

A home sleep test or a split-night study cannot be used in place of this overnight study.^[1]

How the MSLT works

• 4 or 5 naps — the standard test has 5 naps, each starting 2 hours after the last one began. A 4-nap test may be used only when the results already clearly point to narcolepsy.^[1]
• Short naps — if you do not fall asleep, the nap ends after 20 minutes. If you do, the nap ends 15 minutes after you fall asleep.^[1]
• Awake between naps — you stay out of bed and awake between naps. You may have a light breakfast and lunch at set times.^[1]

Plan for a full day at the sleep center, from the night before until late afternoon.

What MSLT results mean

The sleep doctor looks at two things:

• Mean sleep latency — the average time it took you to fall asleep across all naps.
• SOREMPs — sleep-onset REM periods, meaning REM sleep that starts within 15 minutes of falling asleep. Most people do not reach REM sleep that fast.^[2]

For narcolepsy, the MSLT usually shows a mean sleep latency of 8 minutes or less and 2 or more SOREMPs. A SOREMP on the overnight study the night before can count as one of the two.^[2]

A result is never read alone. Not getting enough sleep, shift work, some medicines and stopping some medicines can all change MSLT results. The doctor puts the test together with your symptoms and history. For more on symptoms, see narcolepsy symptoms and diagnosis.

How the MWT works

• 4 wake trials — each lasts up to 40 minutes, spaced 2 hours apart.^[1]
• Sitting up in dim light — you sit in a bed or reclining chair in a quiet, dim room and try to stay awake without tricks like fidgeting or singing.^[1]
• Overnight study is optional — whether you have an overnight sleep study the night before an MWT is up to your doctor.^[1]
• Who gets it — people whose job or driving depends on staying alert, such as some commercial drivers or pilots, and people with narcolepsy or sleep apnea who want to see how well treatment works.^[3]

Staying awake for the full time on every trial is the most reassuring result. AASM practice parameters treat an average time to fall asleep under 8 minutes as abnormal, while results in between are harder to interpret.^[3] The MWT is one piece of a safety decision, not the whole answer. The doctor also looks at your history and how well you use your treatment.^[3]

How to prepare for an MSLT or MWT

• Keep a regular sleep schedule — AASM guidance asks for a sleep diary, and a wrist activity monitor when possible, for 2 weeks before the test.^[1] This shows you have been getting enough sleep. Read about the wrist monitor in actigraphy sleep testing.
• Review medicines with your doctor — medicines that make you alert or sleepy, or that change REM sleep, often need to be tapered at least 2 weeks before an MSLT, and sometimes longer.^[1] Do this only with your doctor's plan. Never stop a medicine on your own.
• A urine drug screen may be done — your sleep doctor may order one on the test day, because some drugs and substances can change results.^[1]
• Skip caffeine, alcohol and nicotine — follow your sleep center's instructions. Caffeine is usually avoided for the MSLT. For the MWT, ask your doctor what is allowed.^[1]
• Bring quiet things to do — a book or puzzle helps you stay awake between naps.

MSLT for children and teens

Narcolepsy often starts in the teen years, so children and teens sometimes need an MSLT. AASM guidance for children recommends at least 7 hours of sleep on the overnight study before the MSLT, and ideally the amount of sleep their age needs.^[4] The test may be adjusted for young children, children with developmental needs and teens who naturally fall asleep late. There is not yet a standard MWT protocol for children.^[4]

When to see a sleep specialist

Ask about daytime sleep testing if you or your child:

• Feel very sleepy during the day — even after a full night of sleep, for weeks or months.
• Fall asleep at the wrong times — in class, at work, during meals or while driving.
• Have muscle weakness with emotion — such as knees buckling or the face drooping when laughing.
• Need to prove alertness for work or driving — your employer or doctor may ask for an MWT.

Daytime sleep testing in Cedar Park

Dr. Minal Patel is board-certified in pediatrics and sleep medicine and cares for children and adults. Our practice offers in-lab sleep studies and narcolepsy evaluation with MSLT testing. We are accepting new patients. Office hours are Monday through Friday, 8:30 to 5:00. We accept Medicaid and CHIP for sleep patients. We do not take walk-ins.