Sleep Calculator
Find the best bedtime or wake-up time based on 90-minute sleep cycles, or work backward from a target number of hours of sleep.
How to use this calculator
- 1Choose whether to calculate by sleep cycles (a table of options between 3 and 6 cycles) or by a specific target duration.
- 2Choose whether you know your wake-up time or your bedtime, and enter that time.
How the calculation works
Bedtime = Wake-up time − (cycles × 90 min) − 15 min. Wake-up = Bedtime + (cycles × 90 min) + 15 min- Sleep cycle
- A full cycle through the stages of sleep, averaging about 90 minutes
- Fall-asleep time
- Average time to actually fall asleep after getting into bed, about 15 minutes
A full night is typically 4 to 6 complete cycles. Waking up at the end of a cycle (light sleep) tends to feel far more refreshing than waking mid-cycle (deep or REM sleep), even for the same total number of hours.
Worked example
Need to wake up at 7:00 AM
- 1.6 cycles = 9 hours of sleep, plus 15 minutes to fall asleep = 9 hours 15 minutes before 7:00 AM.
- 2.7:00 AM − 9 hours 15 minutes = 9:45 PM.
Result: Bedtime around 9:45 PM for a full 6-cycle night
Exactly 8 hours of sleep, waking at 7:00 AM
- 1.8 hours of sleep plus 15 minutes to fall asleep = 8 hours 15 minutes before 7:00 AM.
- 2.7:00 AM − 8 hours 15 minutes = 10:45 PM.
Result: Bedtime around 10:45 PM for exactly 8 hours
What happens across a night’s sleep
Sleep is not a single uniform state — it moves through distinct stages across a night, generally grouped into light sleep, deep (slow-wave) sleep, and REM (rapid eye movement) sleep, when most dreaming occurs. These stages cycle in a repeating pattern lasting roughly 90 minutes on average, several times over a typical night. The mix shifts as the night goes on: earlier cycles tend to contain more deep sleep, while later cycles lean more heavily toward REM.
Why waking mid-cycle can feel worse than the clock suggests
Being woken during deep sleep tends to produce a groggy, disoriented feeling sometimes called sleep inertia, which can persist for a while after waking. Waking during lighter sleep — which occurs more often near the boundary between cycles — is generally associated with feeling more alert. That difference is the premise behind timing an alarm for the end of a full cycle rather than for a fixed number of hours: two nights of similar total sleep can feel quite different depending on which sleep stage the alarm happens to interrupt.
How much sleep is generally recommended
Sleep needs vary by age. Widely cited public-health guidance puts the typical adult range at about 7 to 9 hours a night, with somewhat more commonly recommended for teenagers and considerably more for young children, whose sleep needs decline gradually through childhood. These are population ranges rather than a fixed target every individual must hit — some people function well toward the shorter end, and consistently needing much less than the typical range to feel rested is uncommon.
Habits commonly associated with better sleep
Sleep researchers and public-health bodies commonly point to a handful of habits that support consistent, higher-quality sleep, independent of any particular bedtime calculation:
- A consistent schedule — going to bed and waking at similar times daily, including weekends, helps keep the body’s internal circadian clock aligned.
- Limiting caffeine and alcohol later in the day — both can interfere with the ability to fall or stay asleep, even when consumed several hours beforehand.
- A cool, dark, quiet environment — commonly cited as supportive of falling and staying asleep.
- Winding down before bed — reducing bright light and screen exposure in the period before sleep is frequently recommended, though its individual effect size is debated.
What this assumes, and where it stops
Assumptions
- Sleep cycles average 90 minutes — real cycle length varies by person and by night, typically 70 to 120 minutes.
Limitations
- This is a population-average model, not a measurement of your actual sleep cycles — a sleep tracker gives a more personalised picture.
Common questions
Why 90-minute cycles?
A full sleep cycle — moving through light sleep, deep sleep and REM sleep and back — averages roughly 90 minutes in adults, repeating 4 to 6 times a night. It is an average, not a fixed constant; individual cycles genuinely vary from about 70 to 120 minutes and shift somewhat across the night.
Is more sleep cycles always better?
Generally yes up to your body's natural need — most adults do best with 7 to 9 hours (roughly 5 to 6 cycles) — but timing your alarm for the end of a cycle matters too. Waking after 5 complete cycles can feel better than waking mid-way through a 6th, even though the 6-cycle night is technically more total sleep.
Should I use sleep-cycles mode or target-duration mode?
Use cycles mode if avoiding mid-cycle grogginess matters most to you — it gives several timing options, each a whole number of 90-minute cycles. Use duration mode if you have a specific number of hours in mind (say, exactly 7.5 for a work night) and want the bedtime or wake-up time that hits it precisely, even if that is not a whole number of cycles.
Sources
- Sleep basics — US National Heart, Lung, and Blood Institute
Formula and content last reviewed on .
Results are estimates for information only, not professional advice.
Related calculators
Tools people commonly use alongside the sleep calculator.