Detailed definition
Slow-wave sleep (SWS), formerly called stages 3 and 4 and now combined into N3, is the deepest stage of non-REM sleep, defined by high-amplitude (>75 µV) delta waves (0.5–2 Hz) occupying at least 20% of the EEG epoch. SWS dominates the first 1–3 cycles of the night. Functions associated with SWS include declarative (factual) memory consolidation, growth hormone release, glymphatic clearance of metabolic waste from the brain (including beta-amyloid), and physical/immune recovery. SWS naturally declines with age — by mid-life it may be only 5–10% of total sleep, compared to 20%+ in young adults. Estrogen and progesterone both modulate SWS, with progesterone's allopregnanolone metabolite enhancing SWS via positive allosteric modulation of GABA-A receptors. This is the mechanism by which bedtime oral micronized progesterone improves perceived sleep quality even when total sleep time changes little.
Why it matters in menopause
Loss of slow-wave sleep is a major reason menopausal women report "I slept 7 hours but I'm wrecked." Memory consolidation and metabolic clearance both depend on SWS. The quickest practical lever for restoring SWS in menopausal sleep is bedtime oral micronized progesterone, which often produces noticeable improvement within days.
Related terms
Sources
External references: Wikipedia.