Sleep runs in cycles of roughly 90 minutes, moving between light sleep, deep sleep and dreaming. Waking at the end of a cycle feels very different from being pulled out of deep sleep.
This calculator works backwards from your wake-up time: it shows the bedtime window for your age group and the times that complete whole numbers of sleep cycles.
For educational purposes only — not medical advice.
How to use
- Enter the time you need to wake up.
- Pick the age group that applies to you.
- Press Compute to see your bedtime window and cycle options.
- Count back five minutes for the time it normally takes you to fall asleep.
- Keep the same wake time on weekends where you can — it stabilizes your body clock.
Frequently asked questions
How many hours of sleep do I need?
Guidelines suggest 7–9 hours for adults, 8–10 for teenagers and 9–11 for school-age children. Individual needs vary within those bands.
What is a sleep cycle?
A cycle is one full pass through light sleep, deep sleep and REM sleep, lasting about 90 minutes. You complete roughly four to six cycles a night.
Why count 90-minute blocks?
Waking at the end of a cycle usually feels easier than being woken from deep sleep, so aligning your alarm with cycles can reduce morning grogginess.
Should I add time to fall asleep?
Yes. Most people take 10–20 minutes to drop off, so allow for that when you pick your bedtime, and adjust if your own experience differs.
Is it bad to sleep in on weekends?
A large weekend lie-in shifts your body clock later and creates a Monday morning that feels like jet lag. Keeping within an hour of your weekday time helps.
Can I catch up on lost sleep?
Partly. One recovery night can repay some of the debt, but consistently short sleep is better fixed by moving your bedtime earlier than by relying on catch-up.