How to use the sleep calculator

Pick a mode. “Wake up at” counts backwards from your alarm to suggest bedtimes. “Going to sleep at” counts forwards. “Nap now” plans a short sleep from this moment.
Set the time. The forward modes start at the current time, and the “Use now” button resets them whenever you like.
Adjust the fine-tuning if you want. The fall-asleep time defaults to 15 minutes and the cycle length to 90, which you can move anywhere between 80 and 110.
Read the cards. Each one shows a time, how many complete cycles it gives you, how much sleep that is and how long you would be in bed. The five- and six-cycle options are highlighted as the usual adult target.
Switch to a 12-hour clock if that is how you read times, and copy the whole set with one button.

How the maths works

Everything is plain clock arithmetic. In “wake up at” mode:

bedtime = wake time − (cycles × cycle length) − time to fall asleep

and in “going to sleep at” mode the same equation runs the other way:

wake time = bedtime + time to fall asleep + (cycles × cycle length)

Input	Default	Range
Cycle length	90 min	80–110 min
Time to fall asleep	15 min	0–120 min
Cycles shown	3, 4, 5, 6	4.5 h to 9 h of sleep

Times are handled as minutes past midnight and wrapped back into a 24-hour clock, so a bedtime that lands before midnight is labelled as the night before and a wake time after midnight as the next day. A 6-cycle bedtime for a 07:00 alarm comes out at 21:45 on the previous evening; a 4-cycle bedtime for the same alarm is 00:45 on the morning itself.

Worked examples

A 07:00 alarm. Six cycles is 540 minutes of sleep. Add the 15-minute fall-asleep allowance and you need 555 minutes in bed, so lights out at 21:45. Five cycles gives 23:15, four gives 00:45 and three gives 02:15. The 23:15 option is the one most adults will actually use: 7.5 hours of sleep, 7 hours 45 in bed.

Going to bed at 23:00. Add 15 minutes to fall asleep and the first cycle starts at 23:15. Three cycles ends at 03:45, four at 05:15, five at 06:45 and six at 08:15. If your alarm is set for 06:30, the five-cycle option at 06:45 tells you that you are waking 15 minutes early — which is usually worse than it sounds, because that quarter of an hour is likely to be REM sleep.

A longer cycle. Set the cycle to 100 minutes and ask for five cycles before a 07:00 alarm: 500 minutes of sleep plus a 30-minute fall-asleep time puts bedtime at 22:10. Anyone who consistently wakes feeling mid-cycle at the 90-minute suggestions is worth trying this on.

An afternoon nap at 14:00. With a 15-minute latency the power nap ends at 14:35, a 30-minute nap at 14:45, an hour at 15:15 and a full cycle at 15:45. The short options are far easier to wake from.

Recommended sleep by age

These are the general, widely published ranges used by most national health bodies. They describe populations, not individuals.

Age group	Recommended sleep
👶 Newborn (0–3 months)	14–17 h
🍼 Infant (4–11 months)	12–15 h
🧸 Toddler (1–2 years)	11–14 h
🎨 Preschool (3–5 years)	10–13 h
🎒 School age (6–13 years)	9–11 h
🎧 Teen (14–17 years)	8–10 h
💼 Adult (18–64 years)	7–9 h
📻 Older adult (65+)	7–8 h

Tips and common mistakes

Cycle length is an average, not a law. Nobody’s sleep runs on a metronome, and cycles lengthen as the night goes on. Treat every suggested time as approximate to within ten or fifteen minutes.
Consistency beats cleverness. The same bedtime and wake time every day, weekends included, does more for how rested you feel than hitting an exact cycle boundary once.
Do not confuse time in bed with sleep. The cards separate the two deliberately: eight hours in bed with a 30-minute latency is seven and a half hours of sleep.
Late naps borrow from the night. A long nap after mid-afternoon reduces the sleep pressure that helps you fall asleep in the evening.
Waking a little early is not a bonus. If your alarm sits 20 minutes before a cycle boundary, moving it earlier to the previous boundary often feels better than the extra sleep.

Glossary

Sleep cycle – one pass through light sleep, deep sleep and REM, roughly 90 minutes.
Sleep latency – the time between lights-out and being asleep.
Sleep inertia – the groggy, heavy feeling of being woken out of deep sleep.
REM – rapid eye movement sleep, where most vivid dreaming happens; its share grows through the night.

About these suggestions

This is a screening and planning tool, not a diagnosis; talk to a healthcare professional if you regularly struggle to fall asleep, wake unrefreshed, snore heavily or feel sleepy during the day. The page offers no medical advice and no sleep treatment — it does clock arithmetic around a widely used average, entirely in your browser. Nothing you type is uploaded, logged or stored.

Frequently asked questions

How long is a sleep cycle?

Around 90 minutes on average, but genuinely variable — roughly 80 to 110 minutes between people, and cycles tend to lengthen through the night as REM takes a larger share. That is why the cycle length is adjustable here rather than fixed at 90.

Why does the calculator add 15 minutes before the first cycle?

Because falling asleep is not instant. The time between lights-out and actually being asleep is called sleep latency, and around 15 minutes is a commonly used average. If you habitually drop off faster or take much longer, change the number and every suggestion shifts with it.

Is five or six cycles better than seven?

Five or six cycles is 7.5 to 9 hours at a 90-minute cycle, which covers the range usually recommended for adults, so those options are highlighted. Seven cycles is not harmful — it is simply more sleep than most adults need, and the point of the tool is to avoid waking mid-cycle rather than to cap your sleep.

Does waking at the end of a cycle really feel better?

The idea is that waking during light sleep is easier than waking out of deep sleep, which is where the groggy feeling known as sleep inertia comes from. It is a reasonable rule of thumb rather than a guarantee, since nobody can predict exactly when your cycles will end.

How long should a nap be?

Short naps of 10 to 20 minutes usually end before deep sleep and are easy to wake from. Naps of 30 to 60 minutes often run into deep sleep and can leave you groggy. A full 90-minute cycle brings you back to light sleep, which is why it is offered as the longest option.

How much sleep do I actually need?

Most adults are advised to aim for 7 to 9 hours, teenagers 8 to 10 and school-age children 9 to 11. These are general population ranges, not personal prescriptions — some people genuinely do well on a little less, and some need more.

Related tools

Last reviewed: September 21, 2026

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