The short answer for most adults: 7 to 9 hours. But that range hides important variation — and both too little and too much sleep carry health consequences. Here's what the research actually says.
| Age Group | Recommended Hours |
|---|---|
| Newborns (0–3 months) | 14–17 hours |
| Infants (4–11 months) | 12–15 hours |
| Toddlers (1–2 years) | 11–14 hours |
| Pre-school (3–5 years) | 10–13 hours |
| School age (6–13 years) | 9–11 hours |
| Teenagers (14–17 years) | 8–10 hours |
| Young adults (18–25) | 7–9 hours |
| Adults (26–64) | 7–9 hours |
| Older adults (65+) | 7–8 hours |
Source: National Sleep Foundation recommendations
Sleep isn't a single state — it progresses through 90-minute cycles, each consisting of lighter sleep stages followed by deep slow-wave sleep and REM sleep. A full night of 7.5 hours contains roughly 5 complete cycles.
Waking in the middle of a cycle — especially during deep sleep — causes that heavy, groggy feeling (sleep inertia). Waking at the natural end of a cycle feels much better. This is why sleeping 7.5 or 9 hours often feels more refreshing than 8 hours exactly.
Enter your wake-up time and the calculator will work out the best bedtimes based on complete 90-minute sleep cycles.
Open Sleep Calculator →Consistent sleep deprivation — even mild, chronic undersleeping at 6 hours instead of 8 — has measurable effects:
Partially. Recovery sleep does restore some cognitive function lost to short-term deprivation. But you can't fully repay a chronic sleep debt — the cellular and metabolic effects accumulate. Sleeping extra on weekends is better than nothing, but it doesn't undo a week of under-sleeping.