Sleep Calculator

Work out the best bedtime or wake-up time based on 90-minute sleep cycles, so you wake up between cycles rather than in the middle of deep sleep.

How it works

  1. Enter either your intended bedtime or the time you need to wake up.
  2. The tool lists recommended alarm or bedtime options aligned to full 90-minute sleep cycles.
  3. Pick the option that gives you the most complete cycles before your wake-up time.

The sleep-cycle model

Sleep moves through repeating cycles of light sleep, deep sleep and REM sleep. The calculator uses a cycle length of 90 minutes, a commonly used average. Waking at the end of a cycle, during lighter sleep, is generally easier than being woken from deep sleep, which can leave you groggy for a while (sleep inertia). The tool therefore offers three whole-cycle options: 6 cycles (9 hours), 5 cycles (7.5 hours) and 4 cycles (6 hours).

It also allows for the time it takes to drop off. By default the tool assumes 14 minutes, a commonly cited average, and you can switch it to 10, 20 or 30 minutes. In Wake at mode it works backwards from the time you must wake: the time you should be asleep is the wake time minus the cycles x 90 minutes, and the time you should be in bed is that minus your fall-asleep time. In Sleep at mode it works forwards: the best alarm times are bedtime plus fall-asleep time plus whole cycles.

A worked example

Say you must be up at 07:00 and take 14 minutes to fall asleep. Six cycles means being asleep by 22:00, which means being in bed at 21:46. Five cycles means asleep by 23:30 (in bed at 23:16), and four cycles means asleep by 01:00 (in bed at 00:46). The last-coffee times, which sit 6 hours before each sleep time, are 16:00, 17:30 and 19:00.

In Sleep at mode, if you get into bed at 23:00 you will be asleep at 23:14, and the cycle-aligned alarm times are 08:14 for six cycles, 06:44 for five and 05:14 for four.

The nap checker

In Nap mode, enter how long you can nap and the tool classifies it. Up to 20 minutes is a power nap that stays in light sleep. From 21 to 45 minutes you are likely to enter deep sleep without finishing a cycle, which is where grogginess is most likely. From 46 to 84 minutes is still risky, and 85 to 100 minutes is close to one complete 90-minute cycle.

How far to trust it

Ninety minutes is an average. Individual cycles vary in length, between people and across the night, so the suggested times are a good starting point rather than a precise prediction, and you may find that one option suits you better than the others. How much sleep you need matters more than when you wake in the cycle: sleep medicine guidance for adults is generally seven or more hours a night, so the six-hour option here is a fallback for a short night, not a target.

The six-hour coffee cutoff reflects caffeine's half-life, which is commonly cited as around five to six hours but varies a lot between people, and is longer in pregnancy and with some medications. This tool is not a diagnostic. If you regularly struggle to fall asleep, stay asleep, or feel unrefreshed despite enough hours, speak to a doctor.

Frequently asked questions

Why 90 minutes specifically?

A full sleep cycle — light sleep through REM — averages about 90 minutes for most adults, though it can range from 70 to 120 minutes for a given individual.

How many cycles should I aim for?

Five to six full cycles, or 7.5 to 9 hours, is the typical target for adults, but the ideal number varies by person and age.

Sources and further reading

Formulas checked against the sources below. Last reviewed . General information only, not professional advice.