😴 Sleep Calculator

How the Sleep Calculator Works

During the night you move through repeating sleep cycles that each last roughly 90 minutes on average. Waking up at the end of a cycle, during lighter sleep, usually feels easier than being woken from deep sleep in the middle of one.

Tell the calculator when you need to wake up and it suggests bedtimes that allow 6, 5, or 4 full cycles (9, 7.5, or 6 hours of sleep), plus the time it typically takes to fall asleep. Or tell it when you are going to bed and it suggests wake-up times. Cycle length varies from person to person (roughly 70–120 minutes), so treat the times as a starting point and adjust to what feels right for you. Most adults are advised to get 7 or more hours of sleep per night.

Formula

Bedtime   = wake-up time − (cycles × 90 min) − time to fall asleep
Wake time = bedtime + time to fall asleep + (cycles × 90 min)
Cycles suggested: 6 (9 h), 5 (7.5 h), 4 (6 h)

How to Use It

1. Choose whether you know your wake-up time or your bedtime.
2. Enter the time.
3. Adjust how long it usually takes you to fall asleep (15 minutes is typical).
4. Pick one of the suggested times — 5 or 6 cycles is ideal for most adults.

Frequently Asked Questions

How long is a sleep cycle?

A full sleep cycle — moving through light sleep, deep sleep, and REM sleep — lasts about 90 minutes on average, although it can range from roughly 70 to 120 minutes and varies through the night and between people.

How many hours of sleep do adults need?

Health authorities such as the American Academy of Sleep Medicine recommend that adults get 7 or more hours of sleep per night on a regular basis. Five cycles (7.5 hours) or six cycles (9 hours) fits that guidance.

Why do I feel groggy even after enough sleep?

Waking during deep sleep can cause sleep inertia — a groggy feeling that can last from minutes to an hour. Timing your alarm to the end of a cycle may help, but consistent sleep and wake times matter more. Persistent tiredness is worth discussing with a doctor.

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Formula and content last reviewed: September 2026. All calculations run in your browser — nothing you enter is sent to or stored on our servers.