Sleep unfolds in 90-minute cycles. Waking at the end of a cycle — not the middle — means you surface from light sleep instead of deep sleep. That's the difference between jumping out of bed and hitting snooze.
This calculator adds a 45-minute onset buffer — the time it realistically takes to wind down and drift off. Your bedtime target already includes that buffer.
Each cycle moves through four stages. Early cycles are loaded with deep (N3) sleep for physical repair. Later cycles shift to REM for memory, emotions, and creativity. Cutting sleep short mostly steals REM.
| Group | Age | Hours |
|---|---|---|
| Toddler | 1–2 yr | 11–14 h |
| Preschool | 3–5 yr | 10–13 h |
| School-age | 6–13 yr | 9–11 h |
| Teen | 14–17 yr | 8–10 h |
| Adult | 18–64 yr | 7–9 h |
| Older Adult | 65+ yr | 7–8 h |
Source: National Sleep Foundation. Individual needs vary.