Sleep Cycle Calculator
Find the best times to go to sleep or wake up in whole sleep cycles.
Results update as you type.
About this calculator
A sleep cycle calculator picks the best times to fall asleep or wake up so you rise at the end of a 90-minute cycle rather than the groggy middle. Sleep runs in these cycles of roughly 90 minutes, and this tool works backward from a wake-up time to suggest bedtimes, or forward from a bedtime to suggest wake times, targeting complete 5–6 cycle nights and allowing time to fall asleep. Times can be shown in 24-hour or 12-hour format.
Choose whether you know your wake-up time or your bedtime, then the tool steps in 90-minute cycles from that anchor and lists the times that complete a whole number of cycles. It also adds a buffer (15 minutes by default) for the time it takes to actually drift off, so the cycle clock starts when you fall asleep, not when you get into bed. It highlights the 5- and 6-cycle options — about 7.5 to 9 hours — as the best targets.
For example, to wake at 7:00 am with a 15-minute fall-asleep buffer, good bedtimes are about 9:45 pm (6 cycles) or 11:15 pm (5 cycles), because each leaves you finishing a cycle right at 7:00 am. Working the other way, going to bed at 11:00 pm points to waking near 6:30 am or 8:00 am. Use it to plan a bedtime for an early alarm, or to pick a wake time that avoids the groggy feeling of waking mid-cycle.
Frequently asked questions
How long is one sleep cycle?
About 90 minutes on average, moving through light, deep and REM sleep. The calculator aims for whole cycles — typically 5 or 6, i.e. 7.5 to 9 hours — so you wake between cycles rather than mid-cycle.
Why add time to fall asleep?
You do not fall asleep the instant your head hits the pillow. The calculator adds a buffer (15 minutes by default) so the cycle timing is measured from when you actually drift off, not when you get into bed.
What time should I go to bed to wake up at 6 am?
Set the mode to wake at a fixed time, enter 6:00 am, and the tool lists bedtimes that finish a whole cycle by then — typically around 8:45 pm or 10:15 pm once the fall-asleep buffer is included.
How many hours of sleep is best?
Most adults do well on five or six complete cycles, which is about 7.5 to 9 hours. The calculator flags those options; younger people often need more.
Is the 90-minute cycle exact for everyone?
No. Ninety minutes is an average — real cycles vary from roughly 80 to 100 minutes and change through the night, so treat the suggested times as a helpful guide rather than a precise rule.
Can it tell me when to wake up if I sleep now?
Yes. Switch to the bedtime mode, enter the time you will go to bed, and it lists wake times at the end of each cycle so you can pick one that fits your morning.
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-cycle/curl
curl "https://calculator.free/api/v1/sleep-cycle/?mode=wake&time=07:00"JavaScript fetch()
const r = await fetch(
"https://calculator.free/api/v1/sleep-cycle/?" + 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.