This content was created by the National Sleep Foundation

NATIONAL SLEEP FOUNDATION — The National Sleep Foundation’s “Sleeptionary” is an evolving list of common sleep terms and their definitions.

Arousals An abrupt change from “deeper” stage of non-REM (NREM) sleep to a “lighter” stage, or from REM sleep toward wakefulness, with the possibility of awakening at the final outcome. May be accompanied by increased tonic electromyographic activity and heart rate, as well as by an increased number of body movements.
Awakenings The return to the PSG-defined awaked state from any NREM to REM sleep stages. Characterized by alpha and beta  elextroencephalographic activity, a rise in tonic electromyographic activity, voluntary rapid eye movements and eye blinks. This definition is valid on insofar at the PSG is paralleled by a resumption of reasonably alert state of awareness of the environment.
Sleep efficiency The proportion of sleep in the episode potentially filled by sleep (i.e., the ratio of TST to TIB).


Fit bit calculation: time asleep / (total time in bed – time to fall asleep). Once you start sleep tracking, the time it takes for you to actually fall asleep will not be incorporated into this calculation.

REM (called  “sleep stage REM” in ICSD) The stage of sleep with the highest brain activity, characterized by enhanced brain metabolism and vivid hallucinatory imagery or dreaming. There are spontaneous rapid eye movements, resting muscle activity is suppressed, and awakrening threshold to nonsignificant stimuli is high. The electroencephalogram is low-voltage, mixed-frequency, nonalpha record. REM sleep is usually 20% to 25% of TST. Also called “paradoxical sleep.”
Deep sleep A common term for combined NREM stage 3 and 4 sleep. In some sleep literature, the term deep sleep is applied to REM sleep because during REM sleep, individuals have a high awakening threshold to nonsignificant stimuli.
Delta sleep Electroencephalographic activity with a frequency of less than 4 Hz (usually .1-3.5 Hz). IN the scoring of human sleep, the minimum characteristics for scoring delta waves are conventionally 75 µV (peak-to-peak) amplitude and .5-second duration (2 Hz) or less.
Sleep patterns A person’s clock-hour schedule of bedtime and arise time as well as nap behavior; the sleep pattern may also include time and duration of sleep interruptions.
Sleep architecture Represents the cyclical pattern of sleep as it shifts between the different sleep stages, including non-rapid eye movement (NREM) and rapid eye movement (REM) sleep. It allows us to produce a picture of what sleep looks like over the course of a night, taking into account various depths of sleep as well as arousals to wakefulness. Sleep architecture can be represented by a graph called a hypnogram.
Sleep stages Distinctive stages of sleep, best demonstrated by PSG recordings the electrocephalgram, electrooculogram, and electromyogram.
Sleep latency The duration of time from “lights out,” or bedtime, to the onset of sleep.
Total Sleep Time (TST) The amount of actually sleep time in a sleep episode; this time is equal to the total sleep episode less the awake time.  TST is the total of all REM and NREM sleep in a sleep episode.
Time in Bed (TIB) The time in the study from `Lights Out’ to `Lights On’.
Sleep quality One’s satisfaction of the sleep experience, integrating aspects of sleep initiation, sleep maintenance, sleep quantity, and refreshment upon awakening; does not have a clear definition
Light sleep A common term for NREM sleep stage 1, and sometimes stage 2.
Sedentary time (called “into-bed time “ in ICSD) The clock time at which a person gets into bed. The into-bed time (IBT) will be the same as the bedtime for many people but not for those who spend time in wakeful activities in bed such as reading, before attempting to sleep.