Sleep hours calculator
Put in when you went to bed and when you woke up. You get three numbers: hours in bed, hours actually asleep, and how many 90-minute cycles that comes to. The last one explains why some seven-hour nights feel fine and others feel like four.
7h in bed, minus 14 minutes to fall asleep. Under the 7–9 hours recommended for adults.
Your alarm lands 46 minutes into cycle 5, the part most likely to leave you groggy.
Three different answers to the same question
Ask how long you slept and there are really three answers. Time in bed is the simple clock difference: 10 PM to 5 AM is seven hours. Time asleep is smaller, because it takes most adults 10 to 20 minutes to drop off and nobody sleeps through without a few brief wakings. Cycles are the number that tracks how you feel. Seven hours in bed comes out at about 4.5 cycles, which means the alarm goes off in the middle of one.
Why the half cycle matters
Waking in the deep part of a cycle produces sleep inertia: the heavy, slow first half hour where your reaction time and judgement are measurably worse, in some lab tests as bad as after a full night without sleep (Hilditch & McHill, 2019). So a 6 hour 46 minute night that ends mid-cycle can feel worse than a 6 hour night that ends cleanly. The calculator flags this and offers the nearest alarm times on either side.
Using it the other way round
This tool looks backward at a night you already had or have already planned. To plan around a fixed time instead, use the bedtime calculator when your wake time is set, or the wake-up calculator when your bedtime is. If the totals keep coming up short across the week, the sleep debt calculator adds them up.
Frequently asked questions
If I sleep at 10 PM and wake up at 5 AM, how many hours is that?
Seven hours in bed. Allowing the average 14 minutes to fall asleep, you are actually asleep for about 6 hours 46 minutes, which is 4.5 sleep cycles. That means the 5 AM alarm lands halfway through a cycle. Setting it for 4:14 AM or 5:44 AM would catch you between cycles instead.
How many hours of sleep do I get from 4:40 AM to 8:50 AM?
4 hours 10 minutes in bed, or roughly 3 hours 56 minutes asleep after a typical 14-minute wind-down. That is about 2.6 cycles: a usable nap-length block, not a night. If you have to sleep in that window, 7:54 AM (two full cycles) is the cleaner wake time.
How do I calculate hours slept across midnight?
Count to midnight, then add the morning. 11:30 PM to 6:15 AM is 30 minutes plus 6 hours 15 minutes, so 6 hours 45 minutes in bed. The calculator does this automatically for any pair of times.
Is time in bed the same as time asleep?
No, and the gap is bigger than most people think. You lose the minutes it takes to fall asleep, plus any time awake during the night. For healthy sleepers the total gap is usually 5–15% of time in bed. If it is more than 15%, the sleep efficiency calculator will show where the time is going.
How many hours of sleep is enough?
For adults 18–64, the recommended range is 7–9 hours of actual sleep; for 65 and over, 7–8 hours. Teenagers need 8–10 and school-age children 9–12. These are averages, so the real test is whether you wake without an alarm and stay alert through the afternoon.