Sleep Cycle Calculator
Find the best time to go to bed or wake up by aligning with your natural 90-minute sleep cycles. Free, no account required, and nothing leaves your device.
Assumes ~14 min to fall asleep — each sleep cycle is 90 min.
Suggested wake-up times
5:40 PM
6 cycles · 9h
4:10 PM
5 cycles · 7h 30m
2:40 PM
4 cycles · 6h
1:10 PM
3 cycles · 4h 30m
11:40 AM
2 cycles · 3h
10:10 AM
1 cycle · 1h 30m
Not medical advice. Sleep cycle length varies between individuals and throughout the night. These times are estimates based on an average 90-minute cycle. Consult a healthcare professional for personalised sleep guidance.
How It Works
Choose your mode
Pick "When to wake up" to find ideal bedtimes, or "When to go to bed" to find the best alarm times for a given bedtime.
Enter your time
Type your target wake-up or bedtime. The calculator pre-fills the current time so you can adjust with a single tap.
Aligned to cycles
Each suggested time marks the end of a complete 90-minute cycle. Waking at a cycle boundary feels more refreshing than mid-cycle.
Pick your window
Highlighted options show the ideal 4–6 cycle windows (6–9 hrs). Choose the one that fits your schedule and aim to wake at that exact time.
Why align your alarm with sleep cycles?
Sleep is not a single uniform state. Your brain cycles through four stages roughly every 90 minutes: light sleep (N1), consolidated light sleep (N2), deep slow-wave sleep (N3), and REM. Waking during deep sleep (N3) triggers sleep inertia — the heavy, foggy feeling that can last up to an hour. Waking at or near the end of a cycle, when you are already in the lightest stage, feels far more natural and energising.
How many hours of sleep do you need?
Most adults need 4–6 complete cycles. Common targets:
| Cycles | Total sleep time | Best for |
|---|---|---|
| 6 cycles | 9 hours | Recovery, illness, teenagers, athletes in training |
| 5 cycles | 7.5 hours | Most adults — the widely recommended baseline |
| 4 cycles | 6 hours | Short-term minimum; not sustainable long-term |
| 3 cycles | 4.5 hours | Emergency only — expect significant next-day impairment |
The 90-minute sleep cycle model
Research by Kleitman and Dement in the 1950s identified the roughly 90-minute ultradian rhythm of human sleep. Each night contains 4–6 of these cycles. The early cycles contain proportionally more N3 deep sleep; later cycles are dominated by REM — the stage critical for memory consolidation, learning, and emotional regulation. Consistently cutting sleep short tends to steal REM disproportionately.
Sleep cycle calculator reference table
Suggested bedtimes to wake feeling refreshed at common alarm times (includes 14-minute sleep-onset estimate):
| Wake-up time | 6 cycles (9 hrs) | 5 cycles (7.5 hrs) | 4 cycles (6 hrs) |
|---|---|---|---|
| 5:00 AM | 7:46 PM | 9:16 PM | 10:46 PM |
| 6:00 AM | 8:46 PM | 10:16 PM | 11:46 PM |
| 6:30 AM | 9:16 PM | 10:46 PM | 12:16 AM |
| 7:00 AM | 9:46 PM | 11:16 PM | 12:46 AM |
| 7:30 AM | 10:16 PM | 11:46 PM | 1:16 AM |
| 8:00 AM | 10:46 PM | 12:16 AM | 1:46 AM |
Features
- Two modes: find the best bedtime for a target wake-up, or the best alarm for a target bedtime.
- Sleep-onset adjustment: adds 14 minutes to account for the time it takes to fall asleep.
- Ideal windows highlighted: 4, 5, and 6-cycle options are marked so you can see the best choices at a glance.
- Your data stays private: all calculations run locally in your browser — nothing is uploaded or stored.
- No account required: open the page and start calculating straight away.
Frequently Asked Questions
What is a sleep cycle?
A sleep cycle is one complete pass through the stages of sleep: light (N1/N2), deep slow-wave (N3), and REM. Each cycle lasts roughly 90 minutes. A healthy night contains 4–6 complete cycles.
Why does waking mid-cycle feel so bad?
Waking from deep N3 sleep causes sleep inertia — grogginess and disorientation that can last up to an hour. Waking near the end of a cycle, during lighter N1/N2 sleep, feels far more refreshing.
How many sleep cycles do I need?
Most adults need 4–6 cycles. Five cycles (7.5 hours) is the widely recommended baseline for adults. Six cycles (9 hours) is ideal when recovering from illness, intense exercise, or sustained sleep debt.
Why does the calculator add 14 minutes?
The average person takes about 10–20 minutes to fall asleep — called sleep-onset latency. Adding 14 minutes to the cycle count ensures your bedtime suggestion is realistic from the moment you lie down, not from when you actually fall asleep.
Are sleep cycles always 90 minutes?
No — 90 minutes is a population average. Cycles can range from 70 to 120 minutes and shift throughout the night. Earlier cycles contain more deep sleep; later cycles contain more REM. These suggestions are useful starting points, not precise prescriptions.
What time should I go to bed to wake up at 6 AM?
For 5 cycles (7.5 hrs): go to bed at approximately 10:16 PM. For 6 cycles (9 hrs): 8:46 PM. For 4 cycles (6 hrs): 11:46 PM. Enter 06:00 in the calculator for instant, personalised results.
Is my data stored or sent anywhere?
No. All calculations happen entirely in your browser using JavaScript. Nothing you enter is ever sent to or stored on any server — not even anonymously.
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