Bedtime options
The formula
How it works
Sleep runs in roughly 90-minute cycles, and waking at the end of a cycle feels more refreshing than waking in the middle of one. Enter your wake-up time or bedtime and the calculator suggests the best times, aimed at full cycles.
FAQ
Why 90-minute cycles?
A full sleep cycle — through light, deep and REM sleep — lasts about 90 minutes on average. Waking at the end of a cycle, rather than during deep sleep, tends to leave you feeling more alert and less groggy.
How many cycles do I need?
Most adults do best on five or six cycles a night, which is about 7.5 to 9 hours. Four cycles (6 hours) is a workable minimum, but fewer than that regularly builds up a sleep debt.
Does the 15-minute fall-asleep buffer apply to everyone?
It is an average — some people drift off in five minutes, others take thirty. If you know it usually takes you longer, aim for an earlier bedtime option than the one suggested.
What if I wake up before my alarm?
Waking naturally a few minutes before an alarm usually means you surfaced at the end of a cycle, which is a good sign. It is generally fine to get up rather than fall back asleep and risk starting a new cycle you will not finish.
Does sleep cycle length change with age?
Cycle length shifts gradually through life — infants cycle faster, while adults settle around 90 minutes. The calculator uses the typical adult average, so results are most accurate for adults and teens.
Can too much sleep also leave me groggy?
Yes — waking in the middle of a cycle after oversleeping can cause the same grogginess as waking too early. That is why the calculator times wake-ups to the end of a cycle rather than simply maximizing total hours.
Should I use the same bedtime every night?
A consistent sleep schedule reinforces your body’s internal clock and generally improves sleep quality more than chasing perfect cycle counts on a single night. Use the calculator as a guide, but consistency matters just as much.
About the sleep calculator
This calculator suggests the best times to fall asleep or wake up so that you rise at the end of a sleep cycle rather than in the middle of one. Sleep is not uniform — it moves through repeating cycles of light, deep and dream sleep, each around 90 minutes long. Waking between cycles, when sleep is lightest, is why the same amount of sleep can leave you refreshed one morning and groggy the next.
How to use it
Choose whether you know the time you need to wake up or the time you plan to go to bed, then enter that time. The calculator lists several options built around whole 90-minute cycles, allowing about 15 minutes to fall asleep first. For example, to wake at 7:00 it might suggest going to bed at around 21:45, 23:15 or 00:45 — each one a whole number of cycles before your alarm, so you wake up naturally at the end of a cycle.
The formula
Each suggested time is a whole number of cycles plus the time it takes to drift off: , where is the number of 90-minute cycles. To find bedtimes, the calculator counts these intervals backward from your wake-up time; to find wake times, it counts forward from your bedtime. The 15-minute buffer accounts for the time most people take to actually fall asleep.
Where it is used
People use it to plan a bedtime that leaves them waking refreshed, or to pick a wake-up time when they have to be up early and want to feel their best. It is popular with shift workers juggling irregular hours, students planning around early starts, and anyone chasing better mornings. While the 90-minute figure is an average and everyone differs slightly, aiming for whole cycles is a simple, practical way to sleep smarter.