Delta waves are classically associated with a state of deep, non-REM restorative sleep, and are the slowest form of electrical activity in the brain. They occur when an individual is deeply relaxed, and are detected on an EEG as electrical oscillations at frequencies of 0.5-4 Hz. They are characteristically smooth, high amplitude waves, which are most often present at frontal or central cortical sites. Delta waves in the brain indicate a deep level of physiological rest and can play a role in mood stabilization, physical recovery, and memory consolidation.

See also: nervous system, brain stem, peak experience, altered state