Find the best times to go to sleep or wake up based on 90-minute sleep cycles.
Sleep is not a single state that starts when you close your eyes and ends when the alarm rings. It cycles through stages — light sleep, deep sleep and REM — in blocks of roughly 90 minutes. Waking up at the end of a cycle, during light sleep, feels very different from being pulled out of deep sleep halfway through one. The groggy, disoriented feeling of a badly timed alarm is real and has a name: sleep inertia.
This calculator assumes each cycle lasts 90 minutes and that it takes about 15 minutes to fall asleep. Neither number is exact — cycles vary from 70 to 120 minutes depending on the person and the time of night, and falling asleep can take anywhere from 5 minutes to an hour. The 90/15 values are the averages used in sleep science, and they work well enough as a planning tool even though they are not a measurement of your actual sleep.
Practically: 5 cycles is 7.5 hours, 6 cycles is 9 hours. Most adults need 7 to 9 hours, so 5 or 6 cycles is the target. Fewer than 4 cycles (6 hours) is consistently associated with impaired cognition and health outcomes, regardless of how fine you feel. Everything runs in your browser — no data is collected.
Bedtime = wake time − (cycles × 90 min) − 15 min to fall asleep. Wake time = now + 15 min + (cycles × 90 min).
That is the average length of a human sleep cycle — from light sleep through deep sleep and REM back to light sleep. The actual length varies from 70 to 120 minutes between people and even between cycles in the same night.
That is the average sleep onset latency for a healthy adult. If you know you take longer or shorter, adjust mentally.
7.5 (5 cycles). Chronic sleep of 6 hours or less is associated with measurable cognitive decline and health risks, even in people who have adapted to it and no longer feel sleepy.
No. If you consistently wake unrefreshed or snore loudly, a medical evaluation is the right step. This is a planning tool, not a diagnosis.
No. The times are calculated in JavaScript and nothing is stored or transmitted.
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