For how we live.
In 425 US adolescents followed with phone sensors and Fitbits across 7,591 nights, heavier-than-usual late-evening phone use tracked with later, shorter sleep — the association growing hour by hour toward midnight.
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.
Short sleep is tied to obesity, and experiments suggest the link is partly causal. Adults told to sleep longer ate about 270 fewer calories a day; dieters who slept less lost far less of their weight as fat.
You cannot turn a healthy immune system up like a dial, and nothing on the supplement shelf has been shown to. What the trials do show is narrower: fix a sleep deficit and you catch fewer colds.
There is no single number. A US expert panel sets wide bands — 12 to 16 hours for infants down to 8 to 10 for teenagers — and warns that regularly falling short is linked to attention, behaviour and learning problems.
A short nap does not wreck most people's night sleep, and can help offset sleep debt. The evidence that gives pause is about long naps: naps of 60 minutes or more track with worse cardiovascular outcomes.
Meta-analyses of more than a million people find a U-shaped link between sleep duration and death, lowest around seven hours. But this is observational, and reverse causation is the hard part to rule out.
One week of five-hour nights cut young men's daytime testosterone by 10–15% in a controlled study. Yet CPAP for sleep apnoea leaves the hormone unmoved, pointing back at body weight.
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.
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.
A UK Biobank analysis of 96,365 people found a dose-response relationship between lifetime night-shift exposure and cardiovascular-kidney-metabolic disease — amplified in those who also slept short.
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.
In 24,223 Swedish workers followed to 2018, insomnia and long sleep raised cardiovascular risk only when accompanied by daytime impairment — and rising sleep duration preceded disease.