Sleep Calculator — What Time Should I Go to Bed?

What time should you go to bed?

Based on 90-minute sleep cycles and about 15 minutes to fall asleep. A general guide for healthy adults - not medical advice. Sleep cycles vary from person to person (roughly 80-120 minutes).


 

Most people count backwards from their alarm in whole hours - 8 hours before 7 AM is 11 PM - and then wonder why they still wake up groggy. Sleep does not work in whole hours; it works in cycles of about 90 minutes, moving from light sleep to deep sleep and back again. Waking up in the middle of a deep-sleep phase feels far worse than waking up with slightly less total sleep at the end of a cycle. This free Sleep Calculator works the other way round: pick your wake-up time and it gives you the bedtimes that land on complete 90-minute cycles, allowing about 15 minutes to fall asleep first.

A worked example: to wake at 7:00 AM, six full cycles (9 hours of sleep) means going to bed at 9:45 PM - 7:00 AM minus 9 hours is 10:00 PM, minus the 15 minutes it takes to fall asleep. Five cycles (7.5 hours) gives 11:15 PM, four cycles (6 hours) gives 12:45 AM, and three cycles (4.5 hours) gives 2:15 AM. The calculator marks the 5- and 6-cycle options as recommended, because most healthy adults need 7-9 hours a night. There is also a "sleep now" mode: tap it and the calculator works forward from the current time, so you can pick the alarm time that ends a cycle instead of interrupting one.

How to use

  1. Enter the time you need to wake up (for example 07:00).
  2. Click "Calculate my bedtimes" - you will see four bedtimes, one per number of sleep cycles.
  3. Pick a bedtime with 5 or 6 cycles where you can - those are marked Recommended.
  4. Already in bed? Click "I want to sleep now" instead and choose an alarm time from the list it shows.
  5. Keep the same wake-up time every day, even on weekends - a steady rhythm makes the cycles work better.

Frequently asked questions

Why 90 minutes per sleep cycle?

A full sleep cycle - light sleep, deep sleep and REM dream sleep - lasts about 90 minutes for most adults, though it varies roughly between 80 and 120 minutes from person to person. Counting in 90-minute blocks is the standard model used by sleep calculators and sleep researchers, because waking at a cycle boundary (light sleep) feels much easier than waking mid-cycle (deep sleep).

Why does the calculator subtract 15 minutes?

Most people need around 10-20 minutes to fall asleep after getting into bed. The calculator assumes 15 minutes, so the bedtime it shows is when to be in bed with the lights out - not when you expect to be asleep. If you usually take much longer to drift off, go to bed a little earlier still.

Is 6 hours of sleep (4 cycles) enough?

For most adults, no - not regularly. Health guidance puts the healthy range at 7-9 hours a night, which is 5 or 6 cycles. Four cycles can get you through a single short night, but making it a habit builds up a sleep debt that affects mood, memory and concentration. Use the 4-cycle option only when a full night is genuinely impossible.

Does this replace medical advice about sleep?

No. This is a general guide based on average sleep cycles. If you snore heavily, stop breathing in your sleep, feel exhausted despite a full night, or cannot fall asleep most nights, talk to a doctor - those can be signs of sleep apnoea, insomnia or another condition a calculator cannot diagnose. Your wake-up time and answers stay in your browser; nothing is uploaded.

Related Tools

More free calculators from ToolVora - every tool runs entirely in your browser, with no sign-up and no uploads: