For how we live.
Yes, within limits. Short sleep is linked to more colds and weaker vaccine responses, but the strongest effects turn up when sleep is measured objectively, not from what people report.
In the largest pooled cohort studies, the lowest-risk amount of sleep sits closer to seven hours, and both too little and too much track higher mortality. But the curve is a warning sign, not a prescription.
In-lab trials that restricted sleep on weekdays and allowed recovery at the weekend found insulin sensitivity and glucose tolerance still worsened. Population surveys point the other way, and the designs explain why.
A laboratory trial found a light-emitting e-reader really does suppress melatonin and delay the body clock. Two 2026 field studies found the day-to-day effect runs almost entirely through when people go to bed.
The number most people quote is not the number the American Academy of Sleep Medicine and Sleep Research Society agreed on. Their recommendation is a floor, set by expert vote rather than by trial.
In a US cohort followed eight years, average weekday sleep rose from 8.1 to 8.3 hours while reports of fatigue and trouble sleeping increased. The socioeconomic gap in duration closed; the gap in symptoms did not.
In a US cohort wearing wrist actigraphs, self-reported sleep exceeded measured sleep by about half an hour, with the gap largest on weekends and among Black adolescents and those from lower-income households.