The U-shaped curve you've probably heard about — with the actual numbers.
Sleep is the base layer under every other health lever — including everything else in this app.
TL;DR
Consistently sleeping under 7 hours or over 9 hours is associated with meaningfully higher all-cause mortality than 7–8 hours.
What the study found
Cappuccio's meta-analysis pooled 16 prospective studies covering over 1.3 million adults. Compared to 7–8 hours, short sleep raised mortality risk by 12% and long sleep by 30%. The relationship held after adjusting for age, sex and cardiovascular risk factors.
How to apply it
Anchor a consistent wake time — that stabilises the whole sleep-wake system. Aim for 7–8 hours of opportunity in bed. If you're regularly sleeping 9+ and still tired, that's a symptom to investigate, not a target.
Caveats
Long sleep is likely a marker of ill health rather than a cause. But short sleep is directly and causally harmful — the interventional evidence for that is unambiguous.
Sleep duration and all-cause mortality: a systematic review and meta-analysis
Cappuccio FP et al. · 2010 · Sleep
Meta-analysis of 16 prospective studies (>1.3M participants): both short (<7h) and long (>9h) sleep durations were associated with significantly increased mortality risk.
Read the full paper