Sleep Calculator
Pick a wake-up time to see when to go to bed — or a bedtime to see when to wake — based on 90-minute sleep cycles.
Times assume an average of 14 minutes to fall asleep and 90-minute cycles. Both are population averages — individual cycles run roughly 70–120 minutes — so treat the results as a starting point, not a prescription. This is not medical advice.
About this tool
Ever slept a full eight hours and still woken up feeling like you were hit by a truck? The culprit is often when the alarm fires rather than how long you slept. Sleep runs in cycles that pass through light sleep, deep sleep and REM, averaging about 90 minutes each, and an alarm that lands in the middle of deep sleep produces that heavy, disoriented feeling known as sleep inertia. This free sleep calculator does the arithmetic for you: enter the time you need to be up and it lists the best times to go to bed for three to six complete cycles, adding the 14 minutes an average adult takes to fall asleep. Flip the mode and it works the other way — tell it when you're going to bed and it suggests wake-up times that land at the end of a cycle.
The 90-minute figure is an honest average, not a law: real cycles range from around 70 to 120 minutes and get longer as the night goes on, so use the results as a sensible starting point and nudge your alarm if you still wake groggy. Everything runs in your browser with no account, no app and no data sent anywhere.
How to use the sleep calculator
- Choose a mode: "I want to wake up at…" or "I'm going to bed at…", then set the time (or press Bed now).
- Read the suggested times — the highlighted 5- and 6-cycle options give 7.5–9 hours of sleep, the range most adult guidelines recommend.
- Set your alarm or head to bed at the time that fits your evening, and adjust by 15–30 minutes if you consistently wake foggy.
Frequently asked questions
Why does the calculator count in 90-minute cycles?
A full night is made of repeating cycles that move through light sleep, deep sleep and REM. Research puts the average cycle at roughly 90 minutes, and waking near the end of a cycle, when sleep is light, tends to feel less groggy than waking from deep sleep in the middle of one. The 90-minute figure is an average: real cycles range from about 70 to 120 minutes and change through the night.
What is the 14 minutes added for?
Nobody falls asleep the instant their head hits the pillow. The average healthy adult takes around 10 to 20 minutes to drift off, so the calculator adds 14 minutes between going to bed and the first cycle starting. If you know you usually take longer or shorter, mentally shift the suggested times by the difference.
How many sleep cycles should I aim for?
Five or six cycles, about 7.5 to 9 hours in bed, lines up with the 7 to 9 hours most adult sleep guidelines recommend, which is why those options are highlighted. Four cycles (6 hours) can work occasionally, and three is only for emergencies — a short night timed to a cycle boundary still leaves you short on total sleep.
Will waking at the end of a cycle really make me feel more rested?
It can reduce sleep inertia, the heavy-headed feeling after waking from deep sleep, but it does not replace getting enough sleep overall. Because cycle length varies from person to person and night to night, treat the times as a sensible starting point and adjust by 15 to 30 minutes if you consistently wake feeling foggy.
Is this tool medical advice?
No. It is a simple arithmetic helper based on population averages and is not a diagnosis or a treatment plan. If you regularly struggle to fall asleep, wake unrefreshed, snore heavily or feel very sleepy during the day, speak to a doctor or sleep specialist rather than relying on a calculator.