HesperaFree web tool · no sign-up
Pick the time you need to wake up and how long you usually take to fall asleep. The calculator counts backwards through full 90-minute sleep cycles, so the alarm lands at the end of a cycle instead of the middle of deep sleep.
Go to bed at one of these times:
Times assume ~90-minute sleep cycles plus the fall-asleep buffer you chose. Real cycles vary from person to person — the Hespera app learns yours from your mornings.
How the numbers are made
The calculator counts backwards from the morning you need, not forwards from an arbitrary bedtime. That is how sleep actually gets planned.
A night is built from roughly 90-minute cycles of light, deep and REM sleep. Waking at the end of a cycle usually feels far better than being pulled out of the middle of one.
Getting into bed and falling asleep are different moments. Most people need 10–20 minutes; stress and late caffeine stretch it further. The calculator adds this buffer for you.
5–6 cycles (7.5–9 hours) suits most adults. The shorter rows exist for honest nights when the full amount is not available — they show the cost instead of hiding it.
Quick reference
Assuming about 15 minutes to fall asleep. Use the calculator above to adjust either number for your own night.
| Wake up at | Bedtime for 9 h (6 cycles) | Bedtime for 7.5 h (5 cycles) |
|---|---|---|
| 5:30 AM | 8:15 PM | 9:45 PM |
| 6:00 AM | 8:45 PM | 10:15 PM |
| 6:30 AM | 9:15 PM | 10:45 PM |
| 7:00 AM | 9:45 PM | 11:15 PM |
| 7:30 AM | 10:15 PM | 11:45 PM |
| 8:00 AM | 10:45 PM | 12:15 AM |
Beyond the average person
A web calculator has to assume an average sleeper. The Hespera iPhone app runs the same countdown on the day you actually had — coffee and its timing, naps, stress, screens, exercise — then hands you a lights-out target, a wake window and a caffeine cutoff, and keeps a ledger of the sleep debt you are carrying. No wearable needed.
FAQ
For a 6:00 AM alarm, go to bed around 8:45 PM for six full sleep cycles (9 hours) or around 10:15 PM for five cycles (7.5 hours), assuming you take about 15 minutes to fall asleep. If you need less winding down, shift those times later by the difference.
For a 7:00 AM alarm, a bedtime around 9:45 PM gives you six sleep cycles (9 hours) and around 11:15 PM gives you five cycles (7.5 hours), including roughly 15 minutes to fall asleep.
Most adults do best on five to six full cycles, which is 7.5 to 9 hours of actual sleep. Four cycles (6 hours) is survivable occasionally but builds sleep debt; three cycles is an emergency measure, not a schedule.
No — 90 minutes is a good average, but real cycles run roughly 80 to 110 minutes and change through the night and between people. That is why this calculator gives you options rather than one “perfect” minute, and why the Hespera app refines the estimate using your own mornings.
Six hours is four full cycles — enough to function, not enough to stay level for long. Most people running on 6 hours accumulate sleep debt across the week and feel it as an afternoon crash. Treat the 4-cycle row as a fallback, not a target.
Because bedtime and sleep time are not the same number. If you get into bed at 11:00 PM but drift off at 11:20, every cycle is shifted 20 minutes. Adding your usual sleep latency keeps the wake-up maths honest.
The web version assumes an average person. The iPhone app runs the same countdown on your actual day — the caffeine you drank and when, naps, stress, screens and exercise — then gives you a lights-out target, a wake window and a caffeine cutoff, and tracks sleep debt over time. No wearable needed.