When should you actually go to bed?
Pick the time you want to wake up. We’ll show you bedtimes that land at the end of a complete 90-minute sleep cycle — so you wake up easy, not groggy.
Go to bed at one of these times
includes ~14 min to fall asleepFull restorative sleep
Recommended for most adults
Acceptable on short nights
Minimum — expect grogginess
Pick a time at the start of a complete cycle to wake feeling rested, not groggy.
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OpenWhy a calculator beats a rule of thumb
The folk rule says eight hours. The biology says five to six complete sleep cycles, ending on light sleep. Those aren’t the same thing.
Waking up at the wrong moment in a cycle — mid-deep-sleep, for example — produces sleep inertia: the heavy, dull, slow feeling that takes 30 minutes to shake. Waking at the end of a cycle, in light sleep, feels almost effortless. That’s the entire value of timing bedtime to cycles instead of to an hours-of-sleep target.
The calculator above takes one input — the time you want to wake up — and works backwards through 90-minute cycles plus a 14-minute buffer to fall asleep. That’s it. The output is a small set of bedtimes that minimize the odds you wake up mid-cycle. Pick the latest one you can manage on the schedule you actually have.
Frequently asked questions
Why 90 minutes per sleep cycle?
A complete sleep cycle — light sleep, deep sleep, then REM — averages about 90 minutes in healthy adults. Waking at the end of a cycle (during light sleep) feels easier than waking mid-cycle (during deep sleep), which is when you feel groggy. The cycle length isn’t exact; it ranges roughly 80–110 minutes and varies by person and across the night, but 90 is the well-established average used in sleep research.
How accurate is this calculator?
It uses a population average. The 90-minute cycle and 14-minute fall-asleep latency are averages from sleep research — your actual values may differ by 10–20%. The calculator is a much better starting point than guessing, but if you keep waking groggy at a "recommended" time, try shifting your bedtime by 15 minutes in either direction for a week.
Should I aim for 6 cycles or 5?
Most adults function best on 5–6 cycles (about 7.5–9 hours of sleep). Six is the safer default. Use 5 cycles only when you can’t get a full night — and watch for sleep debt accumulating across the week. Adults need at least 7 hours; consistently less is linked to higher mortality risk in long-running studies.
What about the 14 minutes to fall asleep?
Healthy sleepers fall asleep in 10–20 minutes on average. We use 14 minutes as a midpoint. If you usually fall asleep faster or slower, the calculator will be off by that difference — go to bed 5–10 minutes earlier if you’re a slow starter.
Is this free? Are there ads?
Yes, free. No signup, no email gate, no popups, and no ads while the site is new. We may add unobtrusive affiliate links to products like blackout curtains and white-noise machines later — these will be clearly labelled and never block the calculator.