“8 hours of sleep is optimal for longevity”

## The Evidence A meta-analysis by Yin et al. in *Sleep Medicine Reviews* (2017) pooling 74 cohort studies and 2.2 million participants found a U-shaped relationship between sleep duration and all-cause mortality: nadir around 7–8 hours, with increased risk both below 6 hours and above 9 hours. Short sleep (<6h) is mechanistically linked to higher cardiovascular risk, metabolic dysfunction (impaired glucose tolerance), immune suppression, and elevated inflammatory markers. It is one of the strongest modifiable risk factors for metabolic syndrome. ## The Long Sleep Problem Long sleep duration (>9h) consistently appears in studies as higher mortality risk, but this is likely bidirectional causality — sick people sleep more, rather than long sleep causing illness. Adjusting for poor health at baseline attenuates (but doesn't eliminate) this association. ## The Real Issue Sleep *quality* may matter as much as duration. Fragmented sleep, sleep apnoea, and non-restorative sleep increase mortality risk even at 8 hours. Tracking sleep architecture (REM %, deep sleep %) likely matters beyond just hours in bed.