Reference

Sleep staging glossary

By InEpoch · Last updated 11 October 2026

This glossary defines the terms used in sleep staging and sleep-study reports, from epochs and hypnograms to sleep stages, the signals used to score them, and the summary measures calculated afterwards. Definitions are written in plain language for technologists, lab managers and researchers.

Autoscoring: Software that automatically proposes sleep stages (and sometimes other events) for a recording, for a person to review.
Confidence (stage probability): The probability a staging model assigns to its chosen stage for an epoch. Low values flag epochs worth reviewing first.
Derivation: A recorded signal defined by the electrodes it compares, such as C4-A1 for a central EEG channel.
EDF / EDF+: European Data Format: a widely used open file format for storing polysomnography and other biosignal recordings. EDF+ adds annotations.
EEG: Electroencephalogram: a recording of the brain's electrical activity from scalp electrodes. The main signal for staging sleep.
EMG (submental): Electromyogram from the chin muscles. Muscle tone is especially low in REM sleep.
EOG: Electrooculogram: a recording of eye movements, important for identifying REM sleep and wake.
Epoch: A 30-second segment of a sleep recording. Each epoch receives exactly one sleep stage.
Hypnogram: A chart of the sleep stage assigned to each epoch across the night.
Inter-scorer agreement: How often different human scorers assign the same stage to the same epoch. It is lowest for N1.
K-complex: A large, brief EEG wave that is a hallmark of N2 sleep.
Lights off / lights on: The times a study's time in bed starts and ends. They are needed for sleep efficiency and sleep onset latency.
N1: The lightest stage of sleep, usually at sleep onset. The hardest stage for both people and software to score consistently.
N2: Stable light sleep marked by sleep spindles and K-complexes. Usually the largest share of the night.
N3 (slow-wave sleep): Deep sleep with high-amplitude slow (delta) waves. Replaces the older stages 3 and 4.
Polysomnography (PSG): An overnight sleep study recording EEG, EOG, EMG and usually breathing, oxygen saturation, heart rhythm and leg movements.
REM sleep: Rapid-eye-movement sleep, with rapid eye movements, very low muscle tone and low-amplitude mixed-frequency EEG.
REM latency: Time from sleep onset to the first epoch of REM sleep.
Sign-off: A reviewer's formal confirmation that a scored study is complete and correct.
Sleep efficiency: Total sleep time divided by time in bed, as a percentage.
Sleep onset latency: Time from lights off to the first epoch of sleep.
Sleep spindle: A short burst of faster EEG activity that is a hallmark of N2 sleep.
Sleep staging: Assigning a stage (Wake, N1, N2, N3 or REM) to every 30-second epoch of a sleep recording.
Total sleep time (TST): Time spent asleep (N1, N2, N3 and REM) during the scored period.
Wake (W): Wakefulness, including awakenings during the night.
WASO: Wake after sleep onset: wake time between the first and last epochs of sleep.

Sources

Related guides

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