Sleep Calculator
Find the best bedtime or wake-up time based on 90-minute sleep cycles.
About the Sleep Calculator
Sleep happens in cycles of roughly 90 minutes, and waking up in the middle of a cycle tends to feel groggier than waking up between cycles. This calculator suggests bedtimes or wake times that align with complete 90-minute cycles, based on a target wake-up or bedtime.
How It Works
For a wake-up time, the calculator counts back in 90-minute cycles and then subtracts the time you usually take to fall asleep; for a bedtime, it adds the time to fall asleep and then counts forward in 90-minute increments (representing 4, 5, and 6 full sleep cycles) to suggest several bedtime or wake-time options that align with completing whole sleep cycles.
Suggested times = Target Time ± (Cycles × 90 minutes), for 4, 5, and 6 cycles, adjusted for time to fall asleep.
Worked example
Scenario: Waking at 7:00 AM and taking about 14 minutes to fall asleep.
- Six 90-minute cycles = 9 hours; 7:00 − 9:00 − 0:14 = 9:46 PM.
- Five cycles = 7.5 hours; 7:00 − 7:30 − 0:14 = 11:16 PM.
- Four cycles = 6 hours; 7:00 − 6:00 − 0:14 = 12:46 AM.
Result: Go to bed at 9:46 PM, 11:16 PM or 12:46 AM.
Tips and common mistakes
- Most adults need 7–9 hours a night; five or six cycles fits most people.
- Cycle length varies from about 70 to 120 minutes, so treat the times as guides.
- A regular wake-up time helps more than any single bedtime.
Assumptions & Limitations
- Assumes an average sleep cycle length of 90 minutes, which is a commonly cited estimate though actual cycle length varies somewhat by individual and by night.
- Includes your typical time to fall asleep in the calculation before counting cycles.
- Sleep cycle length is not perfectly fixed at 90 minutes for everyone — treat the suggested times as a helpful guide rather than an exact science.
Treat sleep times as planning estimates
Sleep-cycle calculators use an estimated cycle length and cannot predict exactly when an individual will enter or leave each sleep stage. Falling-asleep time and sleep needs vary by person and by night, so the suggested times are planning aids rather than medical guidance.
If sleep problems are persistent, severe or affecting daily functioning, a qualified healthcare professional can provide advice based on your circumstances.