Tell it when you have to be up and how many hours you want asleep, and it hands back a lights-out time and a moment to start winding down. Going to bed first? The second tool turns a bedtime into wake-up times. Free, no sign-up, nothing saved.
Sleep calculators
Sleep window calculator
- Lights out by
- 10:30 PM
- Start winding down at
- 9:30 PM
Simple arithmetic, not a prescription. How much sleep people need varies. Use it to plan the window, then write down what actually happens.
Wake-up time calculator
set the alarm for
- 7 hours asleep6:00 AM
- 7.5 hours asleep6:30 AM
- 8 hours asleep7:00 AM
- 8.5 hours asleep7:30 AM
- 9 hours asleep8:00 AM
Asleep at about 11:00 PM. The highlighted row is only the middle of the list, not a target set for you.
Reading the numbers
The hours are the ones you choose. The calculator does not decide how much sleep you need, and it does not know whether you will actually fall asleep on cue.
- Pick a wake-up time you can keep on workdays and days off alike.
- Count back to lights-out, then add the minutes you usually lie awake first.
- Write down what really happened for a week, and compare.
The sleep debt calculator adds those nights up for you.
how it works
Clock arithmetic, nothing hidden.
Your alarm is the anchor
Wake time stays fixed. Everything else is measured back from it, so a late night moves bedtime, never the alarm.
Subtract the hours
Lights-out equals wake time minus the hours you picked. The wind-down mark sits one hour before that.
Allow time to drift off
The wake-up tool adds the minutes you usually need to fall asleep, then adds 7 to 9 hours on top.
lights-out = wake time − hours asleep · wake-up = bedtime + minutes to fall asleep + hours asleep
what the research says
Five findings behind the hours.
- Seven or more hours is the CDC figure for adults aged 18 to 60. It lists 7 to 9 hours for ages 61 to 64 and 7 to 8 hours from 65 on.[28]
- A night is not one even block. We move through non-REM and REM sleep several times, with deep sleep concentrated early and REM periods lengthening toward morning.[4]
- Light reaching the eyes keeps the body clock aligned to the 24-hour day, and light at night can shift sleep timing.[17]
- In one lab study, reading a light-emitting e-reader before bed meant taking longer to fall asleep and a later body clock than reading a printed book.[16]
- Schedule changes, stress, caffeine, alcohol and screens near bedtime can all raise the risk of insomnia.[1]
These are group findings. They describe what is common, not what your body will do on a given night.
Sleep calculator questions
How does a sleep calculator work?
It is subtraction. Start from the time you need to be up, take away the hours you want asleep and the minutes you usually need to drift off, and you have a lights-out time. Run it the other way to turn a bedtime into an alarm.
How many hours of sleep do adults need?
The CDC lists 7 or more hours for adults aged 18 to 60, 7 to 9 hours for ages 61 to 64, and 7 to 8 hours from 65 on.[28] Those are population figures; your own need can sit anywhere around them.
What time should I go to bed if I wake up at 6 AM?
For 7 hours asleep, be asleep by 11 PM; for 8 hours, by 10 PM; for 9 hours, by 9 PM. If you usually need 15 minutes to drift off, get into bed 15 minutes before each of those.
Should I time my alarm to 90-minute sleep cycles?
We don’t build this calculator around a fixed cycle length. The NINDS describes sleep as moving through non-REM and REM stages several times a night, with more deep sleep early and longer REM periods later.[4] It does not give one cycle length that fits everyone, so neither do we.