How the sleep calculator works
Sleep runs in cycles of light sleep, deep sleep and REM that last roughly 90 minutes each in adults (the real range is about 80 to 120 minutes). Waking at the end of a cycle, when sleep is lighter, tends to feel easier than an alarm in the middle of deep sleep.
The calculator shows bedtimes for 3 to 6 cycles. The American Academy of Sleep Medicine recommends that adults get 7 or more hours a night, which means 5 cycles (7.5 hours) or 6 cycles (9 hours). The average person takes 10 to 20 minutes to fall asleep; adjust that field if you know you take longer.
Worked example
To wake at 7:00 AM and allowing 15 minutes to fall asleep, the bedtimes are 9:45 PM for six cycles, 11:15 PM for five and 12:45 AM for four. If you are in bed at 11:15 PM you would be asleep by about 11:30 PM and finish your fifth cycle right around 7:00 AM.
Frequently asked questions
How long is a sleep cycle?
About 90 minutes on average for adults, with natural variation from roughly 80 to 120 minutes and longer REM periods later in the night.
How much sleep do adults need?
The AASM and Sleep Research Society recommend 7 or more hours per night for adults 18 to 60. Teens need 8 to 10 hours.
Is 6 hours of sleep enough?
For most adults, regularly sleeping 6 hours leads to measurable declines in attention and health. Short sleep occasionally is not harmful, but it should not be routine.
Why do I wake up tired after 8 hours?
You may be waking mid-cycle, sleeping poorly due to noise, light or alcohol, or have a sleep disorder such as sleep apnea. Persistent fatigue is worth discussing with a doctor.
Updated October 2026. Results are estimates for education, not financial or medical advice. All math runs in your browser; nothing you type is sent anywhere.