Sleep Calculator
Find the best time to sleep or wake up based on sleep cycles. Calculate optimal bedtime for refreshing sleep and energy.
This sleep calculator works backwards or forwards from a clock time in whole 90-minute cycles, adding 15 minutes to fall asleep. Choose Wake at or Sleep at, set the time, and it lists four options for six, five, four and three cycles - so a 07:00 alarm suggests going to bed at 21:45, 23:15, 00:45 or 02:15, with the six- and five-cycle rows marked recommended.
Wake up at:
💡 Sleep Tips
• Avoid screens 1h before bed
• Keep a consistent sleep schedule
• Keep room temperature cool (18-20°C)
About Sleep Calculator
The arithmetic is deliberately simple and entirely visible. One cycle is treated as 90 minutes and falling asleep as 15 minutes, both fixed. In Wake at mode each suggested bedtime is the alarm time minus cycles times 90 minus 15 more; in Sleep at mode each wake time is your bedtime plus cycles times 90 plus the same 15. Four rows are produced, from six cycles down to three, each labelled with its cycle count and its hours of sleep - 9.0, 7.5, 6.0 and 4.5 - and the six and five-cycle rows carry a recommended mark.
The Sleep now button skips the input: it takes the current clock, adds the 15 minutes, and lists the same four wake times from that moment. Quick time buttons load common alarm times into the field, and everything is computed from your device's own clock and formatted in your locale, so no time zone question arises and nothing is sent anywhere.
The 90-minute figure is a round average, not a measurement of you. Real sleep cycles vary between people and lengthen through the night, and the time it takes to fall asleep varies far more than 15 minutes. That makes the output a set of sensible bedtimes to aim at rather than a prediction of when you will wake refreshed, and the difference between two adjacent rows - an hour and a half - is much larger than the precision the model actually has.
This page performs a time calculation and nothing else. It does not track your sleep, measure stages, score a night, adjust for age, or account for caffeine, shift work, medication or a sleep disorder. It is not medical advice and cannot diagnose anything: persistent trouble sleeping, or daytime sleepiness despite enough hours in bed, is a matter for a doctor rather than a calculator.
Use Cases
How to use
Choose Wake at to work backwards from an alarm, or Sleep at to work forwards from a bedtime.
Set the time in the large time field, or press one of the quick time buttons underneath it.
Read the four rows: six, five, four and three cycles, each with its clock time and its hours of sleep.
Prefer a row marked recommended, which is the six and five-cycle options, if your evening allows it.
Press Sleep now instead when you are going to bed immediately; it counts from the current clock plus 15 minutes.
Remember the 15 minutes to fall asleep is already included, so the bedtime shown is when to be in bed.
Pro Tips
- The 15 minutes to fall asleep is already in every row. Do not subtract it again - in Wake at mode the bedtime shown is when to be in bed, not when to close your eyes.
- Adjacent rows are 90 minutes apart, which is a big step. If none of them suits, move the alarm time itself by a few minutes rather than trying to land between two rows.
- Sleep now uses your device's clock, not the time in the field, so pressing it after midnight gives four wake times counted from now plus 15 minutes.
- Read the hours column, not just the cycle count. Six cycles is 9.0 hours and three is 4.5, and the recommended marks sit on the six and five-cycle rows only.
- Treat the times as targets rather than promises. The model uses one fixed cycle length for everyone, so consistency night to night matters more than hitting a particular row exactly.
Troubleshooting
The suggested bedtime is 15 minutes earlier than my own arithmetic.
That is the built-in allowance for falling asleep. Every row is cycles times 90 minutes plus 15, so the time shown is when to get into bed rather than when to be asleep. Do not subtract the quarter of an hour a second time.
None of the four times suits my evening.
The rows are 90 minutes apart because they are whole cycles, so there is nothing between them. Change the alarm time in the field by 20 or 30 minutes and all four bedtimes move with it, which usually brings one of them into range.
Sleep now gives different times from the ones in the list.
Sleep now ignores the time in the field and counts from your device's clock plus 15 minutes, so it will only match if the field happens to hold the current time. Use one or the other rather than comparing them.
The times look wrong after I changed my device's clock or time zone.
Every calculation uses the browser's local clock, and Sleep now in particular reads it directly. Reload the page after changing the system time or zone so the tool picks up the new value.
Frequently Asked Questions
Work in whole cycles from the moment you are actually asleep. This page uses a 90-minute cycle and allows 15 minutes to drop off, so from a 07:00 alarm it counts back six cycles to 21:45, five to 23:15, four to 00:45 and three to 02:15. Choose Wake at or Sleep at, set the time and read the row that fits your night.
A repeating progression through lighter sleep, deep sleep and REM that runs several times a night. The 90 minutes used here is a round average: cycles vary between people and get longer towards morning, with more REM in the later ones. That is why this tool is a rough planner rather than a measurement.
Most adults are advised to aim for seven to nine hours, which on this page is the five and six-cycle rows - 7.5 and 9.0 hours - and those are the two marked recommended. Needs vary with age and individual, and a calculator cannot tell you yours; it can only show what a given bedtime and alarm add up to.
Waking in the middle of deep sleep tends to feel worse than waking at the end of a cycle, which is the whole idea behind cycle-based bedtimes. But grogginess also follows from irregular hours, alcohol, an unfamiliar room or an underlying sleep problem, and none of those is something a clock calculation can see.
That is nine hours in bed. Allowing the 15 minutes this page uses to fall asleep, it works out at six full 90-minute cycles ending at 06:45, so 07:00 lands just past the end of a cycle. Whether nine hours is enough for you specifically depends on age and individual variation, which this calculation does not model.
Not in this calculation - it only works with the single stretch of time between the two clock times you give it. A long nap does contain cycles of its own, but combining naps and night sleep into a total is beyond a fixed 90-minute model, and this page makes no attempt at it.
It is exactly as accurate as its two assumptions: a 90-minute cycle and 15 minutes to fall asleep. Both are averages, and your own figures may differ by a good deal, which is why the rows are 90 minutes apart. Use it to pick a bedtime, not to predict how you will feel at the alarm.
The arithmetic is the same at any age, but the assumptions are not. Children sleep in shorter cycles and need more total sleep than adults, so the rows here understate what a young child requires. Use published age-based sleep duration guidance for children rather than a fixed adult cycle length.
Wake at takes an alarm time and counts backwards to bedtimes; Sleep at takes a bedtime and counts forwards to wake times. Both add the same 15 minutes for falling asleep, so the two modes are consistent: a bedtime the first mode suggests produces the original alarm in the second.
No. It is a time calculation with no knowledge of you, and it measures nothing. Ongoing difficulty falling or staying asleep, loud snoring with pauses, or sleepiness during the day despite enough hours in bed are reasons to speak to a doctor, not to adjust a bedtime by 90 minutes.