Sleep Calculator
Find the best bedtime for a set wake-up time — or when to wake if you sleep now — using 90-minute sleep cycles plus 15 minutes to fall asleep.
| Cycles | Sleep | Clock time |
|---|---|---|
| 6 cycles | 9 h | 21:45 (previous day) |
| 5 cycles | 7 h 30 min | 23:15 (previous day) |
| 4 cycles | 6 h | 00:45 |
| 3 cycles | 4 h 30 min | 02:15 |
The math uses the 90-minute average cycle plus about 15 minutes to fall asleep. Individual cycles vary from roughly 80 to 120 minutes, so treat these times as targets, not guarantees.
This tool runs entirely in your browser. Your data never leaves your device — it is not uploaded to any server.
How to use
- 1Choose Wake-up time → bedtime, or Sleep now → wake times.
- 2Enter the time you must wake up, or the time you are going to bed.
- 3Pick a row: each one is a whole number of 90-minute cycles, with the sleep duration shown.
Sleep Calculator
Waking up mid-cycle is what makes mornings feel awful, not the total hours slept. This tool plans sleep in whole 90-minute cycles: tell it when you need to wake up and it lists the bedtimes that land an alarm exactly at a cycle boundary — six cycles (9 hours) down to three (4.5 hours), each including about 15 minutes to drift off.
Going to bed right now? The reverse mode adds cycles forward instead: fall asleep at 23:30 and your natural wake points are 01:15, 02:45, 04:15, 05:45, 07:15 and 08:45 — pick the earliest one that still gives you enough rest.
Frequently asked questions
Are sleep cycles really 90 minutes?
90 minutes is the commonly cited average, but individual cycles vary from roughly 80 to 120 minutes, and the same person's cycles differ across a night. That is why the times here are targets, not guarantees: if you still feel groggy after two weeks of cycle-timed alarms, shift your bedtime 15–20 minutes and see whether mornings improve.
Why does the calculation add 15 minutes?
Because the first cycle only starts when you actually fall asleep, and most people take somewhere around 10–20 minutes. The tool budgets 15 minutes between getting into bed and the start of cycle one; if you typically drop off faster or slower, mentally add or subtract the difference.
How many cycles do I need?
Most adults do well on 5 cycles (7.5 hours) and many need 6 (9 hours); 4 cycles (6 hours) works for some people short-term but rarely long-term. Consistency of schedule matters as much as the total — the same 6 cycles at wildly varying bedtimes leaves you worse off.
Related tools
Pace Calculator
Compute running pace per km and per mile from any distance and time, then predict race times with Peter Riegel's 1.06 exponent formula.
All tools →
One Rep Max Calculator
Estimate your one-rep max from any weight-and-reps set with Epley and Brzycki formulas, plus a 1–12 rep training table with percentages of 1RM.
All tools →
Date Calculator
Days between two dates in years, months and days — or add and subtract days from any date — with leap years handled exactly.
All tools →
Age Calculator
Work out an exact age in years, months and days, total days lived, and a live countdown to the next birthday — calendar accurate.
All tools →
Embed this tool
Put this tool on your own site with one line — it runs entirely in the visitor's browser.
<iframe src="https://zoyku.com/en/tools/sleep-calculator?embed=1" width="100%" height="640" style="border:0;border-radius:16px" title="Zoyku — sleep-calculator" loading="lazy"></iframe>