What is a sleep cycle?

A sleep cycle is approximately 90 minutes and includes different stages: light sleep, deep sleep, and REM sleep. Your body naturally completes cycles in roughly 90-minute intervals.

Why does 90 minutes matter?

Waking up at the end of a complete cycle (rather than in the middle) can help you feel more rested. 4-6 complete cycles typically results in 6-9 hours of sleep, which is the recommended range for most adults.

What do the ratings mean?

Ideal = 5-6 cycles (7.5-9+ hours), Good = 4 cycles (~6.5 hours), Short = 3 cycles (~5 hours). Aim for ideal when possible, but any complete cycle is better than waking mid-cycle.

How do I account for the time it takes to fall asleep?

This calculator includes a fall-asleep time (default 15 minutes). If you typically fall asleep faster or slower, adjust the slider to match your experience for more accurate results.

Can I use this if I work night shifts?

Yes! The calculator works for any 24-hour time. Just enter when you want to wake or sleep, regardless of whether it's day or night.

Is this medical advice?

No. This tool provides general sleep optimization guidance based on the 90-minute cycle hypothesis. Sleep needs vary by age, health, and lifestyle. Consult a healthcare provider for personalized sleep advice.

Methodology: Sleep Calculator

Scope

Suggests bed and wake times that align with 90-minute sleep cycles.

Formula

bedtimes = wake_time - n * 90 min - 14 min sleep latency, for n = 5, 6 cycles (7.5h, 9h).

Coefficients

Parameter	Value	Note
Cycle length	`~90 minutes`
Typical sleep latency	`~14 minutes`
Target cycles (adult)	`5–6 cycles`

Data sources

Hirshkowitz M, Whiton K, Albert SM, et al. National Sleep Foundation's sleep time duration recommendations: methodology and results summary. Sleep Health. 2015;1(1):40-43. — PMID 29073412. Source of the 7-9 h adult target.
Consensus Conference Panel; Watson NF, Badr MS, Belenky G, et al. Recommended amount of sleep for a healthy adult: a joint consensus statement of the American Academy of Sleep Medicine and Sleep Research Society. Sleep. 2015;38(6):843-844. — PMID 26039963. AASM/SRS companion consensus to the NSF duration recommendations.
Centers for Disease Control and Prevention — How Much Sleep Do I Need? — CDC public-health summary consistent with the NSF and AASM recommendations.

Assumptions

Ninety-minute cycle length is a population average; individuals vary.

Approximation range

Actual cycle length varies 80–120 minutes between people and across the night.

Limitations

Cannot see shift work, caffeine timing, or sleep disorders.
Does not replace a sleep-medicine consultation for insomnia or OSA.

Reproducibility

Wake 6:30 AM, 5 cycles: bedtime = 06:30 - (5*90+14) min = 10:46 PM.

Change log

2026-04-24: methodology page first published.

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Worked example

Computed by the same engine bundle served at /engines/sleep-calculator.js. Re-runnable: the values below are the literal output of compute(engineInput).

Input

tool: sleep_calculator
wake_time: 06:30
fall_asleep_minutes: 15

Output

windows: [{"bedTime":{"hours":1,"minutes":45},"wakeTime":{"hours":6,"minutes":30},"cycles":3,"sleepHours":4.75,"rating":"short"},{"bedTime":{"hours":0,"minutes":15},"wakeTime":{"hours":6,"minutes":30},"cycles":4,"sleepHours":6.25,"rating":"good"},{"bedTime":{"hours":22,"minutes":45},"wakeTime":{"hours":6,"minutes":30},"cycles":5,"sleepHours":7.75,"rating":"ideal"},{"bedTime":{"hours":21,"minutes":15},"wakeTime":{"hours":6,"minutes":30},"cycles":6,"sleepHours":9.25,"rating":"ideal"}]
fallAsleepMinutes: 15

FAQ

What is a sleep cycle?: A sleep cycle is approximately 90 minutes and includes different stages: light sleep, deep sleep, and REM sleep. Your body naturally completes cycles in roughly 90-minute intervals.
Why does 90 minutes matter?: Waking up at the end of a complete cycle (rather than in the middle) can help you feel more rested. 4-6 complete cycles typically results in 6-9 hours of sleep, which is the recommended range for most adults.
What do the ratings mean?: Ideal = 5-6 cycles (7.5-9+ hours), Good = 4 cycles (~6.5 hours), Short = 3 cycles (~5 hours). Aim for ideal when possible, but any complete cycle is better than waking mid-cycle.
How do I account for the time it takes to fall asleep?: This calculator includes a fall-asleep time (default 15 minutes). If you typically fall asleep faster or slower, adjust the slider to match your experience for more accurate results.
Can I use this if I work night shifts?: Yes! The calculator works for any 24-hour time. Just enter when you want to wake or sleep, regardless of whether it's day or night.
Is this medical advice?: No. This tool provides general sleep optimization guidance based on the 90-minute cycle hypothesis. Sleep needs vary by age, health, and lifestyle. Consult a healthcare provider for personalized sleep advice.