Sleep Calculator
Find out what time to go to bed to wake up rested. A sleep cycle calculator based on 90-minute REM cycles — 4 optimal bedtimes and wake-up times. Free and online.
We add 15 minutes to fall asleep. One sleep cycle is 90 minutes.
How the Sleep Calculator Works
A night's sleep is made up of repeating cycles of about 90 minutes each. Every cycle moves through light sleep, deep sleep and the rapid eye movement phase — REM. If your alarm goes off in the middle of deep sleep, your body experiences it as a jarring interruption — which is why you feel heavy, groggy and drowsy for the rest of the day.
The calculator times your bedtime or wake-up so you surface at the end of a cycle, during light sleep. It also adds 15 minutes to fall asleep, so the suggestions are realistic rather than idealized.
Enter your alarm time and see when to head to bed. Or switch to "I'll go to bed at…" mode to find the best time to get up. Four options span 4.5 to 9 hours of sleep — from the bare minimum to full recovery.
Why Count Sleep Cycles
Wake up refreshed
Waking at the end of a cycle, in light sleep, feels effortless. An alarm during deep sleep leaves you groggy even after 8 hours in bed.
Less time asleep, more rest
7.5 hours that ends on a clean cycle often leaves you more rested than 8 hours of "whatever happens." Shorter sleep, more energy.
A steady rhythm
Going to bed and getting up at the same time keeps your circadian rhythm in sync. Falling asleep gets easier, and waking up feels better.
Frequently Asked Questions
What is a sleep cycle?
A sleep cycle lasts about 90 minutes and includes three phases: light sleep, deep slow-wave sleep and REM sleep, when we have our most vivid dreams. A typical night runs through 4 to 6 such cycles.
Why 15 minutes to fall asleep?
A healthy person falls asleep in 10 to 20 minutes on average. The calculator uses 15 minutes as a sensible middle ground to keep its suggestions realistic. If you know you take longer, head to bed a little earlier.
What if I wake up on my own, without an alarm?
Then use the "I'll go to bed at…" mode — the calculator shows when you'll naturally wake up. If you can't hit a time exactly, pick the one closest to the end of a cycle.
How many hours of sleep are ideal?
Most adults need 7 to 9 hours. The calculator offers 4 options: 4.5 hours is the minimum to function, 6 is enough for an occasional short night, 7.5 is optimal, and 9 is for deep recovery after heavy strain.
Is this some kind of magic?
The method is grounded in research on sleep architecture and circadian rhythms — notably Matthew Walker's "Why We Sleep." The ideal wake-up lands at the end of a cycle, when sleep is shallow. The calculator is simple cycle arithmetic, not a promise of the perfect night.