Sleep Calculator
Find optimal bedtime or wake-up time based on 90-minute sleep cycles.
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Quick Tip
Choose whether you are setting a wake-up time or a bedtime.
About This Tool
Why use Sleep Calculator?
Sleep runs in cycles of roughly 90 minutes, moving from light sleep through deep sleep to REM. Waking at the end of a cycle usually feels easier than waking in the middle of one, which is why this calculator works in whole cycles rather than a flat number of hours.
Tell it when you need to wake and it counts back in 90-minute blocks, adding about 15 minutes to fall asleep; tell it when you are going to bed and it counts forward. Most adults do best on five or six cycles, which is 7.5 to 9 hours. Cycle length varies between people and across the night, so treat the times as a guide rather than a rule.
Key Benefits
Works in both directions
Count back from a wake-up time or forward from a bedtime.
Falling-asleep time included
About 15 minutes is added, because nobody falls asleep the instant the light goes off.
Cycle count shown
Each suggestion says how many cycles it gives, so you can judge the trade-off when time is short.
How to use Sleep Calculator
- 1
Choose whether you are setting a wake-up time or a bedtime.
- 2
Enter that time.
- 3
Read the suggested times, each one a whole number of cycles.
- 4
Aim for five or six cycles on a normal night, and allow a few minutes either side.
Best use cases
Set an alarm that lands at the end of a sleep cycle.
Work out when to go to bed for an early start.
Plan sleep around a night shift or an early flight.
Decide between a short nap and a full cycle.
Sleep Calculator FAQs
How long is a sleep cycle?
About 90 minutes on average, though it ranges from roughly 70 to 120 minutes and tends to lengthen later in the night.
How many hours of sleep do I need?
Most adults need 7 to 9 hours, which is five or six cycles. Teenagers need more, typically 8 to 10.
Why do I feel worse after a long sleep?
Waking in the middle of deep sleep causes grogginess — sleep inertia — regardless of total duration. Ending on a cycle boundary usually feels better.
How long should a nap be?
Either about 20 minutes, staying in light sleep, or a full 90-minute cycle. The 30-to-60-minute range often ends in deep sleep and leaves you groggy.
Related Everyday Tools
More free everyday tools that work well alongside sleep calculator.