What the sleep cycle calculator does
Sleep is not a flat eight hours; it runs in cycles of roughly 90 minutes that move through light sleep, deep sleep and REM. Waking at the end of a cycle, during light sleep, feels far easier than being pulled out of deep sleep mid-cycle — which is why nine hours can leave you groggy and seven and a half can feel refreshing. Tell the calculator when you want to wake up and it lists bedtimes that end on a cycle boundary; tell it when you are going to bed and it lists the best wake-up times. It allows about 15 minutes to fall asleep.
How the times are calculated
Bedtime = wake time − (cycles × 90 min) − 15 min to fall asleep
Wake time = bedtime + 15 min + (cycles × 90 min)
Wake at 06:30:
6 cycles → bed at 21:15 (9 h in bed)
5 cycles → bed at 22:45 (7 h 30 in bed)
4 cycles → bed at 00:15 (6 h in bed)Five or six cycles (7.5–9 hours of sleep) is the target for most adults. Four cycles is a floor for the occasional short night, not a plan.
What happens in a cycle
| Stage | Share of night | What it does |
|---|---|---|
| N1 — light | ≈ 5% | Drifting off; easily woken |
| N2 — light | ≈ 45–50% | Heart rate and temperature drop; memory consolidation begins |
| N3 — deep (slow-wave) | ≈ 15–25% | Physical repair, immune function, growth hormone; hardest to wake from |
| REM | ≈ 20–25% | Dreaming, emotional processing, learning; muscles paralysed |
Early cycles are heavy on deep sleep; later cycles are heavy on REM. Cutting sleep short by two hours therefore removes a disproportionate amount of REM, and an alarm at the 6-hour mark often lands in the middle of a long REM period.
How much sleep do you need?
| Age | Recommended (hours) |
|---|---|
| Newborn (0–3 months) | 14–17 |
| Infant (4–11 months) | 12–15 |
| Toddler (1–2) | 11–14 |
| Preschool (3–5) | 10–13 |
| School age (6–13) | 9–11 |
| Teen (14–17) | 8–10 |
| Adult (18–64) | 7–9 |
| Older adult (65+) | 7–8 |
Figures from the National Sleep Foundation. Individual need varies by about an hour either way; the test is whether you wake without an alarm and stay alert through the afternoon.
Limits of the 90-minute model
- Cycles range from about 70 to 120 minutes and lengthen slightly across the night, so the boundaries drift. The calculator's times are good targets, not exact.
- Fall-asleep time varies. If you routinely take 30 minutes, shift the bedtimes 15 minutes earlier.
- A consistent wake time matters more than any single night's arithmetic. Waking at the same time daily anchors the circadian rhythm and makes falling asleep easier.
- Alcohol, late caffeine, and screens close to bedtime fragment cycles regardless of timing.
Practical use
- Fixed wake time (work, school): pick the bedtime for 5 or 6 cycles and set a wind-down alarm 30 minutes before it.
- Late night, unavoidable: choose 4 cycles rather than 4.5 — six hours ending on a boundary beats six and three-quarters ending in deep sleep.
- Naps: 20 minutes (light sleep only) or a full 90-minute cycle. A 45–60 minute nap ends in deep sleep and produces the worst grogginess.
- Jet lag: use the calculator on destination time from the first night, even if it means a shorter first sleep.