How to use the Sleep Calculator
- Choose whether you know your wake-up time or your bedtime.
- Enter the time.
- Pick one of the suggested times — 5 or 6 cycles is ideal for most adults.
- Optional: compare your actual bedtime and wake time with your chosen sleep duration.
Why sleep cycles matter
Sleep runs in cycles of about 90 minutes, moving through light sleep, deep sleep and REM sleep. Waking from deep sleep causes “sleep inertia” — that groggy, disoriented feeling. Waking at the end of a cycle, during light sleep, feels much easier.
How much sleep do adults need?
Adults need 7–9 hours a night, which is 5–6 full cycles. Teenagers need 8–10 hours. The calculator also adds the average 15 minutes it takes to fall asleep.
Frequently asked questions
Is every sleep cycle exactly 90 minutes?
No, cycles range from about 70 to 120 minutes and vary through the night. Adjust the cycle length if you know yours.
Is 6 hours of sleep enough?
Four cycles (6 hours) can leave you feeling okay short-term, but regularly sleeping less than 7 hours is linked to health problems.