Sleep Cycle Calculator
Find the best bedtimes or wake times based on 90-minute sleep cycles.
Results update as you type.
About this calculator
A sleep cycle calculator suggests bedtimes or wake-up times aligned to 90-minute sleep cycles, so you wake between cycles rather than in the middle of deep sleep and feel more refreshed. It counts back or forward in 90-minute blocks and adds the time it takes to fall asleep. Most adults do best on five to six complete cycles (7.5–9 hours).
Say you must wake at 07:00 and take about 15 minutes to drop off. Six cycles need 6 × 90 = 540 minutes of sleep plus 15 to fall asleep, so you should be in bed by 21:45; five cycles push that to 23:15. Switch to bedtime mode and it counts forward instead, suggesting wake times that land at the end of a cycle rather than mid-cycle.
Use it to pick a bedtime that leaves you waking during light sleep, when you tend to feel less groggy. The highlighted five- and six-cycle options match the 7.5–9 hours most adults need, while the shorter three- and four-cycle rows suit a nap or an unavoidably late night. Cycle length averages 90 minutes but varies between people and across the night, so treat the times as a helpful guide.
Frequently asked questions
How long is a sleep cycle?
About 90 minutes. A full night moves through four to six of these cycles, each passing from light to deep to REM sleep. Waking at the end of a cycle, rather than mid-cycle, tends to feel less groggy.
How many sleep cycles do I need?
Most adults feel best with five to six complete cycles — roughly 7.5 to 9 hours. The calculator highlights those options and lists shorter ones for a nap or a late night.
Why add time to fall asleep?
You do not sleep the instant you lie down. Adding an average 15 minutes of sleep-onset time makes the suggested bedtime line up with when you actually start cycling through sleep.
What time should I go to bed to wake at 7am?
For five or six 90-minute cycles plus about 15 minutes to fall asleep, aim to be in bed around 21:45 for six cycles or 23:15 for five. The tool lists these bedtimes once you enter a 07:00 wake time.
How do I find the best time to wake up?
Switch to bedtime mode and enter when you will fall asleep; the calculator counts forward in 90-minute cycles and suggests wake times that finish a cycle rather than interrupt one.
Is waking between sleep cycles actually real?
Waking during light sleep at the end of a cycle tends to feel less groggy than waking out of deep sleep. Because cycle length varies through the night, the suggested times are a guide rather than an exact alarm.
API — use this calculator from code
Call this calculator as a free JSON endpoint — no key required. Send the field values below as query parameters or JSON. Read the full API docs →
Endpoint
GET https://calculator.free/api/v1/sleep/curl
curl "https://calculator.free/api/v1/sleep/?mode=wake&time=07:00"JavaScript fetch()
const r = await fetch(
"https://calculator.free/api/v1/sleep/?" + new URLSearchParams({
"mode": "wake",
"time": "07:00"
}));
const data = await r.json();
console.log(data.results);Results are estimates for general guidance only, not financial, medical or tax advice.