Sleep Calculator
Find the best bedtime or wake-up time using 90-minute sleep cycles and your fall-asleep time. Free, in your browser.
🔒 Runs entirely in your browser — nothing is uploadedHow the sleep calculator works
Sleep is not one long block. During the night your brain moves through repeating cycles of light sleep, deep sleep and REM sleep, and an average cycle lasts about 90 minutes. Waking at the end of a cycle, when sleep is lightest, tends to feel easier than being woken in the middle of deep sleep. This calculator works in whole cycles so the suggested times land near the end of one.
The maths is simple. For a chosen wake-up time, the bedtime is wake time − (cycles × 90 minutes) − minutes to fall asleep. For a chosen bedtime, the wake-up time is bedtime + minutes to fall asleep + (cycles × 90 minutes). For example, to wake at 7:00 with 15 minutes to fall asleep, six cycles (9 hours) means going to bed at 9:45 PM, five cycles (7.5 hours) means 11:15 PM, and four cycles (6 hours) means 12:45 AM.
How many cycles and how much sleep?
Adults are generally advised to sleep between seven and nine hours a night, which corresponds to roughly five or six cycles. Four cycles, or six hours, is enough to function for a night or two but is often not enough to feel rested over a longer period. The tool highlights five cycles as a practical target and lets you show any combination of four, five and six.
Remember that the 90-minute figure is an average. Your real cycles may be a little shorter or longer, and you may wake briefly during the night without noticing. Treat the times as a good starting point, then adjust by ten to fifteen minutes depending on how you feel.
Tips and a health note
Keeping a consistent schedule, dimming lights in the evening, avoiding caffeine late in the day and keeping the bedroom cool and dark all help you fall asleep faster, which in turn makes the estimates here more accurate. If you need much longer than 15 minutes to fall asleep, increase the value in the calculator so the suggestions stay realistic. This page is for general information only and is not medical advice; if you struggle with insomnia, loud snoring, or daytime sleepiness, speak to a healthcare professional. Everything is calculated locally in your browser and nothing is uploaded.
How to use
- Pick your goalChoose whether you want bedtimes for a set wake-up time, or wake-up times for going to bed now or at a set time.
- Enter the timeSet the wake-up time or bedtime, and adjust how many minutes you usually need to fall asleep.
- Choose cycles and clockSelect which numbers of 90-minute cycles to show (4, 5 or 6) and switch between 12-hour and 24-hour display.
- Pick a timeChoose the suggested time that fits your schedule, ideally the one giving five or six full cycles.