How sleep time is calculated
Your sleep duration is the time from your bedtime to your wake time. If wake time is earlier on the clock than bedtime, the calculator adds 24 hours so it wraps past midnight. From 23:00 to 07:00 is 8 hours.
Worked example: bed at 00:30 and up at 06:45 is 6 hours 15 minutes. Note that this measures time in bed rather than time asleep — most people take 10 to 20 minutes to drop off, so real sleep usually lands short of the figure shown.
The hours until calculator answers the question that usually follows this one.
How sleep cycles work
A full sleep cycle lasts about 90 minutes, moving through light, deep and REM sleep. The badge divides your sleep by 90 to estimate full cycles — 8 hours is about 5 cycles. Waking at the end of a cycle, rather than mid-cycle, tends to feel less groggy.
How to use this calculator
Enter your bedtime and wake time; overnight spans are handled automatically. To convert the decimal hours back to clock format, use the decimal hours to time calculator, or convert the 24-hour entries with the military time converter.
Working backwards is often more useful. Fix the wake time you cannot move, subtract whole 90-minute cycles, then allow about 15 minutes to fall asleep: for a 06:30 alarm that means lights out around 22:45 for five cycles, or 21:15 for six.
Frequently asked questions
- How many hours of sleep from 11 PM to 7 AM?
- From 23:00 to 07:00 is 8 hours of sleep, which is about 5 full sleep cycles.
- How long is one sleep cycle?
- A sleep cycle is roughly 90 minutes, cycling through light, deep and REM stages.
- Does it handle going to bed after midnight?
- Yes. If the wake time is earlier on the clock than the bedtime, 24 hours are added so the duration wraps correctly past midnight.
- How many sleep cycles should I aim for?
- Most adults do well with about 5 cycles (roughly 7.5–8 hours). Waking at the end of a cycle usually feels more refreshing.
- Are sleep cycles really exactly 90 minutes?
- No, 90 minutes is an average. Real cycles run from roughly 70 to 120 minutes and lengthen through the night as REM takes a bigger share, so treat the cycle count as a guide rather than a precise alarm setting.