Slow-wave sleepDeep sleep · N3 · Delta sleep

Slow-wave sleep is the deepest non-REM stage, named for the high-amplitude delta waves that dominate the EEG. It is hardest to wake from, and it is concentrated in the first half of the night.

It matters most for physical recovery. The largest pulses of growth hormone occur during it, tissue repair and protein synthesis are elevated, and the glymphatic clearance of metabolic waste from the brain appears most active. Sleep restriction studies consistently show impaired recovery and performance when it is curtailed.

In practice it declines with age and is reduced by alcohol, late caffeine and evening high-intensity training. Consistent sleep and wake times, a cool dark room and adequate total duration are the interventions with the best evidence — well ahead of anything sold to enhance it.