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Sleep Cycle Calculator

Suggests bedtimes for a given wake-up time, or wake-up times for a given bedtime, so that sleep ends at the boundary of a full sleep cycle (default 90 minutes) after a typical time to fall asleep (default 14 minutes).

When to use

You want to pick a bedtime or alarm time that completes 3–6 whole sleep cycles instead of waking mid-cycle.

Do not use when: You have a sleep disorder or shift-work schedule needing clinical advice; cycle length varies between people and through the night, so treat the times as approximations.

Formula

wake_up_at: bedtime = time − fall_asleep_minutes − n × cycle_minutes; go_to_bed_at: wake time = time + fall_asleep_minutes + n × cycle_minutes; n = 3…6 (4.5–9 h of sleep at 90-minute cycles)

A night's sleep consists of 4–6 cycles of roughly 90 minutes that progress through light, deep and REM sleep; waking at the end of a cycle tends to feel less groggy. Adults are advised to sleep 7–9 hours, i.e. 5–6 cycles.

Inputs

Parameter	Type	Unit	Required	Description
`mode`	enum: wake_up_at \| go_to_bed_at		yes	Whether the given time is the wake-up time or the time you go to bed.
`time`	string		yes	The known wake-up time or bedtime, 24-hour HH:MM.
`fall_asleep_minutes`	integer	min	default 14	Average sleep-onset latency; 10–20 minutes is typical for healthy adults. Range: ≥ 0, ≤ 120
`cycle_minutes`	integer	min	default 90	Length of one sleep cycle; 90 minutes is the usual average (range about 70–120). Range: ≥ 60, ≤ 120

Outputs

Output	Type	Unit	Description
`suggested_times`	string_list		Bedtimes (for wake_up_at, earliest first: 6, 5, 4, 3 cycles) or wake-up times (for go_to_bed_at: 3, 4, 5, 6 cycles), HH:MM.
`sleep_cycles`	number_list		Number of full cycles for each suggested time, in the same order.
`sleep_hours`	number_list	h	Hours of actual sleep for each suggested time.
`schedule`	list		Objects with cycles, time and sleep_hours for each suggestion.

Example

Wake up at 06:30: {"mode":"wake_up_at","time":"06:30"} → {"suggested_times":["21:16","22:46","00:16","01:46"],"sleep_cycles":[6,5,4,3],"sleep_hours":[9,7.5,6,4.5]}

Go to bed at 23:00: {"mode":"go_to_bed_at","time":"23:00"} → {"suggested_times":["03:44","05:14","06:44","08:14"],"sleep_cycles":[3,4,5,6],"sleep_hours":[4.5,6,7.5,9]}

GET https://tttkmbb.com/api/v1/calculate/sleep-cycles?mode=wake_up_at&time=06%3A30

Machine access

API: GET https://tttkmbb.com/api/v1/calculate/sleep-cycles (query parameters) or POST with a JSON body {"inputs": {...}}
Schema: https://tttkmbb.com/api/v1/calculators/sleep-cycles · Markdown: https://tttkmbb.com/everyday/sleep-cycles.md · JSON definition: https://tttkmbb.com/everyday/sleep-cycles.json
MCP: server https://tttkmbb.com/mcp, tool run_calculator with calculator_id="sleep-cycles"
OpenAPI operationId: calculate_sleep_cycle_times
Freshness: static. Authentication: none. Rate limit: fair use (see rate limits).

Sources

NIH NINDS – Brain Basics: Understanding Sleep (government)
Sleep cycle (Wikipedia) (reference)

FAQ

Which suggestion should I choose?

Prefer 5 or 6 cycles (7.5–9 h); 3–4 cycles is a fallback for a short night, not a healthy routine.

Are sleep cycles really 90 minutes?

On average. Individual cycles range from about 70 to 120 minutes and lengthen through the night, so the suggested times are estimates to be fine-tuned by how you feel on waking.

Related calculators

Time Duration Calculator — Hours between bedtime and wake-up time.