A night's sleep consists of 4–6 ultradian cycles (~90 min each). Early cycles are dominated by deep slow-wave sleep (SWS/N3); REM periods lengthen across the night. This architecture is highly conserved across mammals.
Sleep spindles (12–16 Hz, thalamocortical) during N2 and N3 tag synapses for consolidation. Sharp-wave ripples (SWRs) in hippocampal CA1 replay waking sequences, transferring memories to cortex during slow oscillations.
Rapid Eye Movement sleep features a desynchronized EEG resembling wakefulness, muscle atonia, and vivid dreaming. REM promotes emotional memory integration and creative insight. Noradrenergic neurons in locus coeruleus are completely silent.
During SWS, the glymphatic system (named for glia) uses astrocytic AQP4 channels to drive CSF through perivascular spaces, flushing amyloid-β, tau, and metabolic waste at 2× the waking rate. Sleep deprivation → toxic accumulation.