Sleep Definition Terminology

Here are the definitions to some commonly used sleep terms (photo credit:

In our initial post, we talked about common sleep terminology, A-I. In our next post, we covered I – PThis post continues on with that topic, covering sleep terminology you may hear while talking with a sleep professional. This article covers sleep terminology, R – Z.

Rapid-Eye-Movement (REM) Sleep –  This is the sleep stage that is characterized by the rapid eye movement that indicates the presence of powerful brain activity and is most closely associated with dreaming.


Resetting the Circadian Clock – The circadian clock is the internal mechanism that controls the sleep wake cycle and if it gets out of synch with the rhythms of nature, it can make it difficult to sleep at night and remain awake during the day.  It is possible to reset this clock using bright lights that simulate sunshine.


Restless Legs Syndrome (RLS) – This is a sleep disorder that causes those who have it to experience aches, unpleasant sensations, and the uncontrollable need to move in their legs.  Although this can happen at any time during the day, these symptoms are worse at night and can disrupt sleep.


Seasonal Affective Disorder (SAD) – This is a disorder that occurs in some people when they don’t have access to enough sunlight.  It generally occurs in the fall and winter months and causes sadness and depression.


Shift Work – This is the term used to describe a job that occurs during non-regular hours like in the late afternoon and evening or overnight.


Shift Work Disorder – This is a sleep disorder commonly suffered by those who work on alternate shifts that can cause insomnia and excessive daytime sleepiness.


Sleep Apnea – This is a sleep disorder characterized by stops in breathing during sleep.


Sleep Debt – This represents the amount of sleep a person has lost.


Sleep Efficiency – This is the ratio of the total time you spent sleeping to the total amount of time you spent in bed.  The closer the ratio is to 1:1, the better.


Sleep Inertia – This describes the groggy feeling that can happen when you first wake up that can affect everything from cognitive function to physical ability.  For some people, sleep inertia can last for as long as an hour after waking.


Sleep Latency – This is the amount of time it takes from when you begin your potential sleep period i.e. lay down, turn out the light, close your eyes, and when you actually fall asleep.


Sleep Onset – This is the point at which you go from waking to sleeping and enter the first stage of NREM sleep.


Sleep Paralysis – This happens during REM sleep and keeps your body from acting out what is happening in your brain by immobilizing or paralyzing it.


Sleep Spindle – This is a specific type of sleep wave seen on an EEG that indicates Stage 2 NREM sleep.


Stage 1 (N1) Sleep – This is the first and lightest stage of NREM sleep.


Stage 2 (N2) Sleep – This is the second stage of NREM sleep.


Stage 3 (N3) Sleep – This is the deepest stage of non-REM (NREM) sleep, also called slow-wave sleep.


Suprachiasmatic Nucleus (SCN) – This is another name for the circadian clock.


Theta Activity – This is the activity seen on an EEG activity that has a frequency of 4–7 Hz or cycles per second.