A Glossary of Sleep Terms

Sleep disorders can be difficult to understand because of complex medical terminology. This glossary can help.

Many people think getting a good night’s sleep is as easy as putting your head on your pillow and closing your eyes. Those with sleep disorders, however, realize there is much more to it than that, starting with medical terminology that sometimes barely sounds like English. Use this glossary of important sleep-related terms to better understand your condition.

Advanced sleep phase disorder

People with this sleep disorder go to sleep very early, typically between 6 p.m. and 9 p.m., and wake up extremely early, anywhere from 1 a.m. to 5 a.m.

Bright light therapy

Used for jet lag and sleep disorders such as advanced sleep phase disorder and delayed sleep phase syndrome, bright light therapy exposes you to a light much brighter than regular household light; if used at certain times of the day it can help to reset your body clock.


Cataplexy results in a loss of control of one’s muscles. It can happen during emotional situations and also in people with narcolepsy.

Circadian rhythms

Circadian rhythms, controlled by the body’s biological clock, are the changes that occur during the cycle of a day, making you alert in the daytime and tired at night.

Continuous positive airway pressure (CPAP)

Often used to help treat sleep apnea, CPAP uses an air compressor and a mask that covers the nose and mouth. A continuous stream of pressurized air blows into your airway to keep it from collapsing during sleep.

Delayed sleep phase syndrome

This circadian rhythm disorder keeps you from getting sleepy until later at night — often not until the early morning hours — and to sleep later than most people. Individuals with delayed sleep phase syndrome often go to sleep between 1 a.m. and 4 a.m. and wake up anywhere from 8 a.m. to 11 a.m.


This is the recurring inability to fall asleep or stay asleep for a necessary period of time. It affects up to 60 million Americans each year.


This hormone helps regulate biological rhythms including sleep. It is made naturally in the body and is also available as a supplement to help treat insomnia and jet lag.


This sleep disorder causes sudden attacks of deep sleep that last from seconds to more than half an hour. Cataplexy, temporary paralysis, and hallucinations can also occur.

Obstructive sleep apnea

Also called sleep apnea, this disorder causes you to stop breathing while you’re asleep. During deeper phases of sleep, a person with sleep apnea will have periods when the muscles in the back of the throat and around the upper windpipe relax so much that the opening in the airway collapses, leaving you without oxygen for anywhere from 10 seconds to a minute. Eventually the lack of oxygen rouses you enough for the upper airway muscles to tighten and air to flow in again. Often, though not always, sleep apnea is accompanied by snoring. People with this disorder are awakened many times a night, though they are usually not aware of this, and can experience daytime sleepiness and other side effects.

Periodic limb movement disorder (PLMD)

PLMD causes you to move your legs every 20 to 40 seconds, severely disrupting your sleep. People with PLMD may also have restless legs syndrome.

Rapid eye movement (REM) sleep

REM sleep is the stage of deep sleep when your eyes move in a rapid motion and your breathing and heart rate are faster. This is also the stage of sleep when you normally would dream.

Restless legs syndrome (RLS)

This disorder causes a tingling, crawling, or prickling feeling in the legs. During the day, constantly keeping your legs moving can help ease symptoms, but at night, RLS may cause insomnia. A number of people with restless legs syndrome also have periodic limb movement disorder.

By Kristen Stewart – Medically reviewed by Lindsey Marcellin, MD, MPH