Skip to content
Burrows Tools

Sleep Calculator

Give this calculator the time your alarm goes off and it counts backwards in 90-minute blocks, so the alarm lands between sleep cycles rather than in the middle of one. Give it a bedtime instead — or press Now for the time it is right this minute — and it counts forwards to the wake times on the other side. Every suggestion is labelled with the number of cycles it holds and how long you would be in bed, and fifteen minutes are added for falling asleep, because getting into bed and being asleep are not the same moment. Both numbers are yours to change. The 90-minute figure is an average people repeat rather than a fact about you: measured cycles run roughly 70 to 120 minutes and stretch as the night goes on, so read these times as a starting point, not a prescription. The arithmetic is plain clock math done in this tab — no time zone to get wrong, and nothing about your night leaves your browser.

Show times as

6 cycles — 9 hr asleep

9:45 PM (the night before)

9 hr 15 min in bed, including 15 min to fall asleep.

5 cycles — 7 hr 30 min asleep

11:15 PM (the night before)

7 hr 45 min in bed, including 15 min to fall asleep.

4 cycles — 6 hr asleep

12:45 AM

6 hr 15 min in bed, including 15 min to fall asleep.

Go to bed at one of these times to wake at 7:00 AM between cycles rather than in the middle of one. The 90-minute cycle is an average people quote, not a measurement of you — real cycles run roughly 70 to 120 minutes and change length through the night, so treat these as a starting point.

How to use the sleep calculator

  1. Pick a direction: 'I want to wake up at' when the alarm is fixed, or 'I am going to bed at' when the bedtime is.
  2. Set the time, or press Now in bedtime mode to use the current time on your own clock.
  3. Change the minutes-to-fall-asleep buffer if fifteen is not how long it takes you, and the cycle length if you have a better number for yourself.
  4. Read the suggestions: each one shows the clock time, how many cycles it covers, and how long you would be in bed — with a badge when it lands the night before or the next morning.
  5. Switch between 12-hour and 24-hour display if the page guessed the wrong one, then copy the list if you want it somewhere else.

Frequently asked questions

Is the 90-minute sleep cycle real?
It is a popular average, not a law. Sleep genuinely runs in cycles that move through light sleep, deep sleep and REM, and 90 minutes is the round number most often quoted for one of them — but measured cycles vary roughly 70 to 120 minutes between people, and they get longer and more REM-heavy as the night goes on. A calculator built on a flat 90 minutes is a rule of thumb dressed up as arithmetic. That is why the cycle length is an input here: change it to whatever fits you, and trust how you feel over what any page tells you.
Does waking between cycles actually stop the grogginess?
That is the claim the tool is built on, and it is worth being plain about how strong it is. The heavy, disoriented feeling of being woken out of deep sleep is well described and has a name — sleep inertia — and the idea of timing an alarm to avoid it is popular for that reason. What a fixed 90-minute clock cannot know is where you actually are in a cycle on a given night. Treat this as a scheduling aid rather than health advice: it is a better guess than picking a bedtime at random, and it is not a substitute for talking to a doctor if you are consistently exhausted however you time it.
Why does it add fifteen minutes before the first cycle?
Because falling asleep takes time, and a bedtime that ignores it is wrong by exactly that much. Fifteen minutes is the default because it sits in the middle of the range usually quoted for a healthy adult. If you are out in five minutes, set it to five; if you lie there for forty, set it to forty and watch every suggested bedtime move earlier by the same amount. Set it to zero and the tool does pure cycle arithmetic with nothing added.
How many cycles should I aim for?
The list runs from four to six because that is the band adult schedules land in: five cycles is about seven and a half hours of sleep and six is about nine. Guidance for adults is usually stated as somewhere in the seven-to-nine-hour range, which puts five and six cycles inside it and four (about six hours) below it. Four is on the list because a short night is sometimes the only choice available, not because it is a good target.
Do time zones or daylight saving change the answers?
No, and that is by construction. The tool does clock-face arithmetic in minutes since midnight — it adds, subtracts, and wraps past midnight — and never builds a date, so there is no time zone to get wrong and no daylight-saving transition to fall into. Seven and a half hours before 06:30 is 23:00 on every clock on earth. The one night that is genuinely different is the night your local clocks change, when an hour is added or removed and no arithmetic on this page can know it. Nothing you enter is uploaded, and the page keeps working offline once it has loaded.

Related tools