Sleep Calculator
Find the best times to go to sleep based on 90-minute sleep cycles and your wake-up time.
Last updated · View algorithm, constants and default inputs (JSON)
About this calculator
This calculator works backward from your wake-up time using fixed 90-minute blocks plus your estimated time to fall asleep. Sleep cycles vary (often about 80–100 minutes), so these are schedule examples, not a way to guarantee feeling rested. Sleep needs also vary by age; adults 18–60 are generally advised to get at least 7 hours.
Worked example
calculated with the default values below
Wake-up hour 7 h · Wake-up minute 0 min · Minutes to fall asleep 15 min 6 cycles (9h) 21:45 · 5 cycles (7h 30m) 23:15 · 4 cycles (6h) 00:45 · 3 cycles (4h 30m) 02:15
General background references
Disclaimer
These calculators are for general information only and are not a medical device, a diagnosis or a treatment plan. Reference ranges describe populations, not individuals, and do not account for pregnancy, medication, disability or athletic training. Discuss your own situation with a qualified healthcare professional.
Implementation source
This record describes the implementation used by this page. It is not independent validation. External references, when available, support only the stated formula or definition; estimates and custom rules retain their own assumptions.
Formula and variables
B_n = [W − (90n + t)] mod 1440, n = 3…6
- W
- Wake time (minutes of the day)
- n
- Number of 90-min cycles
- t
- Minutes to fall asleep
- B_n
- Suggested bedtime (minutes)
Each cycle is 90 minutes; the value wraps modulo 1440 and is shown as HH:MM; five cycles give 7.5 h of sleep.
Frequently asked questions
How are the suggested bedtimes calculated?
The tool subtracts 90 minutes per selected cycle plus the entered fall-asleep delay from the wake time. For a 07:00 alarm and 15-minute delay, five cycles produce 23:15 the previous night, and six cycles produce 21:45. The 90-minute cycle is a planning assumption; sleep timing and quality vary by person.