Sleep Calculator
Choose whether you want to wake up at a set time or are going to sleep at a set time, then enter that time. For a wake-up time the calculator estimates the bedtimes for 6, 5 or 4 sleep cycles. For a bedtime it estimates the wake-up times after 4, 5 or 6 cycles. Each cycle counts as 90 minutes, and every time includes 15 minutes to fall asleep.
How to use the Sleep Calculator
- Enter your numbers in the Sleep Calculator above.
- Press the Calculate Sleep Times button to see the result.
- To try other numbers, change any value and press the button again.
The sleep cycle math
Bedtime = Wake time − (N × 90 min + 15 min) · · · Wake time = Bedtime + N × 90 min + 15 min
N is the number of cycles, 4, 5 or 6. The calculator counts every cycle as exactly 90 minutes and adds 15 minutes to fall asleep, so 5 cycles mean 7.5 hours asleep and 7 hours 45 minutes in bed. Both numbers are fixed assumptions, and your real cycles differ. The calculator counts clock minutes and ignores the date and your time zone, so on a night the clocks change, the real time in bed is an hour longer or shorter than it shows.
Worked example
To wake at 7:00 AM after 5 cycles (7.5 hours): bedtime = 7:00 AM − 7 h 30 min − 15 min = 11:15 PM. For 6 cycles (9 hours) the bedtime is 9:45 PM, and for 4 cycles (6 hours) it is 12:45 AM. Going to sleep at 11:00 PM gives 5:15 AM after 4 cycles, 6:45 AM after 5 and 8:15 AM after 6.
Frequently asked questions
Is every sleep cycle 90 minutes long?
No. The National Heart, Lung, and Blood Institute says the cycle starts over every 80 to 100 minutes, usually four to six times a night. The calculator uses a flat 90 minutes for every cycle, so its times are estimates.
Will waking at the end of a cycle make me feel more rested?
This calculator cannot promise that, because it cannot see when your cycles end. At 80 to 100 minutes a cycle, five real cycles take 400 to 500 minutes, while the calculator counts 450. Its wake-up time could miss the end of your fifth cycle by up to 50 minutes in either direction. Use the list to plan how long you sleep.
How much sleep do I need?
The CDC lists 8 to 10 hours a day for teens aged 13 to 17, 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. Four cycles give 6 hours of sleep, under all of those adult figures. Five cycles give 7.5 hours and six give 9 hours.
Why does the calculator add 15 minutes?
It allows 15 minutes to fall asleep in every option. That is a round figure of this calculator, not a measurement. If you usually fall asleep faster or slower, move the time by the difference.