Sleep science
Brain waves during sleep: an overview
What the frequency bands are, what they are not, and where each one appears across a night. Each band has its own page.
5 min readbeginnerStrictly reviewed · cited
EEG measures electrical activity at the scalp, conventionally divided into frequency bands. A caution before the list, because it is the single most abused idea in popular sleep material:
These are descriptive categories, not switches, states or resources. The brain does not run "in delta". Delta is a shape that appears in a recording. A great deal is sold on the premise that these are dials you can turn.
The bands, and where they belong
| Band | Frequency | Where it dominates |
|---|---|---|
| Delta | 0.5 to 4 Hz | Deep non-REM, first half of the night |
| Theta | 4 to 8 Hz | Drowsiness and light non-REM; hypnagogic imagery |
| Alpha | 8 to 13 Hz | Relaxed waking, eyes closed; disappears at sleep onset |
| Beta | 13 to 30 Hz | Active, alert waking |
| Gamma | above 30 Hz | Contested; increased frontally in lucid REM |
REM does not belong on that table
REM is defined by a combination, desynchronised mixed-frequency EEG, muscle atonia, and the eye movements it is named for, rather than by a band. Placing it on the delta-to-gamma scale is a category error, and popular material makes it constantly. It has its own page.
How a night moves through them
Alpha gives way to theta at sleep onset. Delta builds through the first half. REM periods lengthen through the second half, looking on the EEG far more like waking than like the deep sleep an hour earlier.
Which is why the two halves of a night are not interchangeable, and why cutting sleep short removes dreaming disproportionately.
Sleep-science pages summarise published research for general education. They are not a substitute for a clinician. If a sleep problem is affecting your health, speak to a doctor, and never change prescribed medication because of something you read here.
Sources and reviewed references
3 references. What these labels mean
- 1.Über das Elektrenkephalogramm des MenschenHistorical dream record
Hans Berger (1929) · Archiv für Psychiatrie und Nervenkrankheiten
Cited for: The first recording of the human EEG, and the identification of the alpha rhythm: the origin of every frequency band later named.
- 2.The AASM Manual for the Scoring of Sleep and Associated Events: Rules, Terminology and Technical SpecificationsSleep medicine
Conrad Iber and colleagues, for the American Academy of Sleep Medicine (2007) · American Academy of Sleep Medicine
Cited for: The current clinical standard for how sleep stages are scored, including the EEG frequency criteria that define each stage.
- 3.Regularly Occurring Periods of Eye Motility, and Concomitant Phenomena, During SleepPeer-reviewed research
Eugene Aserinsky and Nathaniel Kleitman (1953) · Science
Cited for: The discovery of REM sleep and its association with dream recall, the founding observation of modern sleep science.