What Is a Sleep Cycle?
Sleep occurs in recurring stages across the night: lighter sleep, deeper non-REM sleep, and REM sleep. These stages form sleep cycles. Cycle length and composition vary throughout the night and between people — they are not identical fixed-length blocks.
This calculator uses a configurable average cycle length (default 90 minutes) as a planning estimate only.
Why Does the Calculator Use 90 Minutes?
90 minutes is a common planning assumption for a sleep cycle, but real cycles vary. You can change the cycle length (70–120 minutes) rather than treating 90 minutes as a biological constant. Waking near the end of a planned cycle may feel better for some people, but total sleep duration still matters more than hitting an exact boundary.
How the Bedtime Calculator Works
Bedtime = Wake time − (cycle length × cycles) − fall-asleep time
Example: wake at 7:00 AM, 5 cycles of 90 minutes (7h 30m), 15 minutes to fall asleep → bedtime 11:15 PM.
Sleep Duration by Age
Recommended sleep ranges change with age. Approximate guidance used by this calculator (aligned with common public-health ranges such as CDC/AASM summaries):
- School-age (6–12): 9–12 hours
- Teen (13–18): 8–10 hours
- Adults: At least 7 hours (often 7–9)
- Older adults (65+): 7–8 hours
A cycle estimate should not be used to justify getting less sleep than the recommended duration for your age group.
Sleep Cycles vs Sleep Duration
A useful schedule depends on how long you sleep and how that time is structured. Do not sacrifice total sleep only to land on a convenient cycle boundary.
Circadian Rhythm
The circadian rhythm is the body’s approximately 24-hour timing system that helps regulate sleep–wake patterns. This calculator can show a planned daily sleep window on a 24-hour timeline. It cannot measure melatonin, cortisol, core body temperature, or your individual circadian phase.
ADHD and Circadian Rhythm
Sleep timing and circadian patterns can be relevant for some people with ADHD, but this tool does not diagnose ADHD-related sleep problems or calculate a unique “ADHD circadian phase.” You can still use it to plan consistent bed and wake times and compare sleep duration. Persistent sleep difficulties warrant discussion with a qualified healthcare professional — not an online calculator.
REM Sleep
REM is one stage of sleep; REM periods often become more prominent later in the night. This calculator does not detect when you enter REM. Any cycle/REM illustrations are general educational models, not measurements of your sleep stages.
Common Mistakes
- Assuming every cycle lasts exactly 90 minutes — it is an estimate.
- Choosing too little sleep because it matches a cycle — duration still matters.
- Treating calculated REM times as measured REM — a clock-based tool cannot stage sleep.
- Ignoring fall-asleep time — time in bed ≠ time asleep.
- Using one sleep target for every age — needs change across the lifespan.
- Confusing circadian rhythm with sleep cycles — related but different concepts.