How to use
Choose what you know: the time you need to wake up or the time you’re going to bed. The calculator suggests a bedtime or alarm time so that you wake at the end of a sleep cycle rather than in the middle of one. Going to bed right now? Choose “Go to bed at” — the current time is filled in.
How it’s calculated
Sleep runs in cycles of roughly 90 minutes: light sleep, deep sleep, then REM (dreaming) sleep. Waking at the end of a cycle usually feels easier than being pulled out of deep sleep — less grogginess.
bedtime = wake-up time − cycles × 90 min − time to fall asleep
Example: you need to be up at 7:00 AM and take 15 minutes to fall asleep. For 5 cycles (7.5 hours) go to bed at 11:15 PM; for 6 cycles (9 hours), at 9:45 PM.
FAQ
How many cycles do I need? Adults are advised to get 7–9 hours — that’s 5 or 6 cycles. Four cycles (6 hours) is a one-off fallback; three is for emergencies only.
Is a sleep cycle always 90 minutes? No, that’s an average: cycles range from about 70 to 120 minutes and vary through the night. If waking is still hard, try moving your alarm 10–15 minutes either way.
Why include time to fall asleep? Cycles start when you’re asleep, not when you lie down. Healthy adults usually fall asleep within 10–20 minutes.
If sleep problems last for weeks, talk to a doctor.