Sleep

A regulated behavioural and physiological state identified through converging criteria including reversibility, responsiveness and homeostatic regulation. Immobility alone does not establish sleep.

Slow-wave sleep is a deep stage of non-rapid eye movement sleep characterized by prominent slow, high-amplitude activity in an electroencephalography recording. It is a physiological classification based on measured brain activity and other sleep criteria, rather than simply an unusually motionless posture. In current human staging it broadly corresponds to N3. ^[ref]

Sleep-stage identification can guide interventions. In a human experiment, an odour associated with earlier learning was presented again during slow-wave sleep; subsequent declarative-memory performance was compared with control conditions. ^[ref] This links a defined physiological state to an experimentally timed cue; it does not establish that every kind of memory depends exclusively on that one stage.

The proposal that waking-associated strengthening is balanced by sleep-associated renormalization of overall strength of synapses. Selective weakening could improve capacity and relative signal contrast while preserving important differences. It is compatible with some strengthening or reactivation, and is not the claim that sleep indiscriminately erases learning.

Combined recording used to classify sleep and waking, commonly including electroencephalography, eye-movement signals and muscle activity. Classification of a sleep stage is not itself a measure of the content of a memory.

A sleep state identified by rapid eye movements and characteristic electrophysiological and muscle-tone patterns. Its contribution to a particular learned task must be tested rather than inferred from the existence of dreams.

A group of sleep stages distinct from rapid eye movement sleep. Deep stages feature prominent sleep slow oscillations; stage assignment uses polysomnography.

A transient thalamocortical oscillatory event, often around 10–16 Hz in humans, with frequency boundaries depending on the measurement convention. Its timing relative to other events can matter beyond its total count.

A slow alternation in cortical population activity, conventionally below about one cycle per second. Its phase can organize other sleep events; a scalp electrical rhythm alone does not reveal a particular represented episode.