TL;DR
The CDC recommends 7–9 hours per night for adults 18–64 and 7–8 hours for those 65+. Sleep is organized into ~90-minute cycles: NREM (including deep sleep) dominates the first half of the night, REM the second half. Caffeine has a 5–6 hour half-life, and intake within 6 hours of bedtime measurably harms sleep quality.
Sleep stages
| Stage | Characteristics | Main role |
|---|---|---|
| N1 | Light sleep, ~5% | Relaxation, easily awakened |
| N2 | Light sleep, 45–55%, sleep spindles | Memory consolidation, recovery |
| N3 | Deep/SWS, 15–25%, early night | Physical repair, growth hormone |
| REM | Rapid eye movement, 20–25%, late night | Dreams, emotional memory integration |
Common disruptors
- Caffeine: 5–6 h half-life; intake within 6 h of bedtime hurts sleep onset and deep sleep;
- Alcohol: helps onset but suppresses REM and causes late-night awakenings;
- Bright screens: suppress melatonin; reduce strong light an hour before bed;
- Irregular schedule: a fixed wake time stabilizes the circadian rhythm more than a fixed bedtime.
Improvement checklist
- Keep a fixed wake time (within 1 h on weekends);
- Bedroom at 18–22°C, dark and quiet;
- No caffeine after 14:00;
- Naps ≤ 20 minutes to avoid waking from deep sleep;
- Insomnia lasting 2+ weeks or severe daytime sleepiness warrants medical evaluation.
FAQ
How much sleep do I need?
7–9 hours for ages 18–64 and 7–8 for 65+. Feeling rested and alert during the day is your personal signal.
How long should a nap be?
About 20 minutes restores alertness without disrupting nighttime sleep; 30–60+ minutes risks deep-sleep inertia.
Can I repay sleep debt?
Only partially in the short term; chronic debt is not erased by weekend lie-ins. A consistent schedule beats catch-up sleep.
Sources
- CDC: How Much Sleep Do I Need?, accessed 2026-08-04
- NIH NHLBI (sleep stages), accessed 2026-08-04
- Sleep Foundation (caffeine and sleep), accessed 2026-08-04