Overview
- The Brain Medicine review published Tuesday combines evidence from studies of hunter‑gatherer groups, pooled analyses and recent organ‑level physiology to challenge a one‑size‑fits‑all eight‑hour target.
- Field studies in Tanzania, Namibia and Bolivia and a broader analysis find typical nightly sleep in pre‑industrial populations ranges from about 5.7 to 7.1 hours with a pooled mean near 6.4 hours.
- A large physiological study using brain and body scans estimated organ‑specific optimal sleep between about 6.4 and 7.8 hours, with variation by sex and by which organ was measured.
- Epidemiological research links both short and long sleep to worse health outcomes but cannot yet show whether long sleep causes illness or instead signals underlying disease.
- Official guidance from the National Sleep Foundation still advises 7–9 hours for most adults while acknowledging individual needs can range more widely, and experts say timing and weekly totals matter as much as nightly hours.