Sleep is one of the few interventions where doing it badly demonstrably accelerates cognitive decline, cardiovascular disease, and all-cause mortality. The evidence base is robust and the action items are surprisingly concrete.
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Conclusion: 7–9 hours, consistent timing, dark + cool room — Level 1 recommendation
[Level 1 (Strongest)] [Lifestyle] [Strongly recommended]
Adult humans need 7–9 hours of sleep per night, with consistent bed and wake times. Both shorter and longer durations correlate with elevated mortality (U-shaped curve). Quality and timing matter as much as quantity.
📊 Why it matters
- All-cause mortality: <6 hours/night → 12% higher mortality; <5 hours → 28% higher (multiple meta-analyses)
- Cognitive function: Chronic sleep restriction below 6 hours degrades attention, memory, and decision-making to a level comparable to legal alcohol intoxication after 2 weeks
- Alzheimer’s risk: Poor sleep impairs glymphatic clearance of amyloid-β; longitudinal cohorts show 1.5–2× higher dementia risk in chronic short sleepers
- Cardiovascular events: Chronic short sleep raises hypertension and CV event risk by 20–40%
🎯 The high-yield sleep hygiene basics
- Consistent schedule: same bed and wake time daily, including weekends. This matters more than most realize.
- Morning light exposure: 10–30 minutes of bright outdoor light within 1 hour of waking anchors circadian rhythm
- Cool room: 16–19°C is the optimal range for most adults
- Dark room: even small amounts of ambient light disrupt melatonin
- Avoid caffeine after 14:00: half-life is ~6 hours; later doses fragment sleep architecture even if you fall asleep fine
- No alcohol within 3 hours of bed: it sedates but suppresses REM
