TL;DR
Sleep is an active process of memory consolidation and metabolic cleanup, not "shutting down." Adults typically need 7–9 hours; quality depends on sleep architecture and circadian timing.
Key Points
- Sleep stages: NREM (light/deep) and REM alternate in ~90-minute cycles, 4–6 cycles per night. Deep sleep aids physical restoration; REM aids memory consolidation and emotional regulation.
- Circadian rhythm: driven primarily by light (especially morning); melatonin rises at night. Regular timing matters more than total duration alone.
- Caffeine: half-life ~5–6 hours; afternoon intake can still impair deep sleep.
- Sleep debt: chronic short sleep accumulates cognitive and metabolic costs; weekend catch-up does not fully reverse it.
- Practical tips: consistent wake time, less evening blue light, cool bedroom (~18–20°C), avoid alcohol before bed (it suppresses REM).
Sources
- Wikipedia: Sleep / Circadian rhythm
- US NIH sleep resources
- Sleep research overviews (e.g., Walker, "Why We Sleep")