Sleep Calculator
Pick whether you want to wake up at a target time or go to bed at a target time. The calculator works backward or forward in 90-minute sleep cycles so you wake at the end of a cycle — not stuck in deep sleep.
Suggested bedtime
11:15 PM
- 4 cycles · 6h sleep
- 12:45 AM
- 5 cycles · 7h 30m sleep
- 11:15 PM
- 6 cycles · 9h sleep
- 9:45 PM
- Time to fall asleep (latency)
- 15 min
Wake at 7:00 AM. Based on a ~90-minute sleep cycle, 5 full cycles gives 7h 30m of sleep — the CDC recommends 7+ hours for adults. Allow ~15 minutes to fall asleep.
How to use this calculator
Choose "I want to wake up at…" or "I want to go to bed at…" and enter the time in 24-hour format (e.g. 07 : 00 for 7am, 22 : 30 for 10:30pm). Adjust the fall-asleep latency if you typically take longer than 15 minutes to drift off. The headline shows the recommended option (5 full cycles, ~7.5 hours sleep) and the breakdown lists 4, 5 and 6-cycle alternatives so you can pick what fits your schedule.
How the calculation works
A typical adult sleep cycle lasts about 90 minutes and runs through light NREM, deep NREM, and REM sleep. You feel groggy when an alarm jolts you out of deep sleep mid-cycle and refreshed when you wake at the end of one. The calculator counts complete 90-minute cycles backward from your target wake time, or forward from your target bedtime, and adds the time it takes you to fall asleep. The CDC recommends 7+ hours of sleep for adults — 5 cycles equals 7.5 hours, which is the default suggested option.
Worked example
You want to wake at 7:00 AM and usually take 15 minutes to fall asleep. Five 90-minute cycles is 450 minutes (7.5 hours). Working backward: 7:00 AM − 7h 30m sleep − 15m latency = 11:15 PM bedtime. If you can stretch to six cycles (9 hours sleep), bedtime is 9:45 PM; if you need to be up earlier and accept four cycles (6 hours), bedtime is 12:45 AM.
Frequently asked questions
Is the 90-minute sleep cycle exactly the same every night?
No — published research puts the average between 70 and 120 minutes, and the duration of each stage within a cycle shifts across the night (more deep sleep early, more REM toward morning). Ninety minutes is the commonly-used average from the Sleep Foundation and the NIH. Use the suggested times as a starting point, then nudge by 15–20 minutes over a week to find what leaves you most refreshed.
Why does the calculator recommend 5 cycles instead of 6 or 8 hours?
Five full cycles equals 7.5 hours of sleep, which matches the CDC and NHS recommendation of at least 7 hours per night for adults aged 18–64. Six cycles (9 hours) suits teenagers, athletes in heavy training, or anyone recovering from sleep debt; four cycles (6 hours) is the absolute minimum many people can function on short-term and is shown for nights when a full 7.5 hours is not possible.
What is sleep latency and why does it matter?
Sleep latency is the time between lying down and actually falling asleep. The healthy range is roughly 10–20 minutes; under 5 minutes can be a sign of sleep deprivation, over 30 minutes can be a sign of insomnia or excessive caffeine. The calculator adds latency to your bedtime calculation so you actually get the full cycles in — not 15 minutes less.
Should I use this for shift work or jet lag?
The cycle math still applies, but circadian alignment matters more for shift workers and travellers crossing time zones. If you are working nights or recovering from a long-haul flight, use the calculator to pick a wake time that gives you 5–6 full cycles in a dark, cool room — but combine it with consistent light exposure on waking and avoiding caffeine within 8 hours of your target bedtime.
Is it bad to wake up mid-cycle if my alarm forces me to?
Not dangerous, but you will feel groggier (this is sleep inertia) and cognitive performance dips for 15–30 minutes after waking. If your fixed wake time falls mid-cycle, shift your bedtime so you complete a cycle instead — even losing 15 minutes of sleep to land on a cycle boundary often feels better than the extra deep-sleep awakening.
Does this replace a sleep tracker?
No. Wearables like Fitbit, Apple Watch and Oura estimate actual cycle timing from heart rate and movement and can wake you at the end of a real cycle within a window. This calculator gives you a planning estimate based on averages — useful for setting a bedtime, not for replacing personalised tracking.