Sleep Calculator
Find the best time to sleep or wake up based on 90-minute sleep cycles. Get 5-6 complete cycles for optimal rest and energy
Based on 90-minute sleep cycles (15 min to fall asleep)
02:15
✗ Minimal (3 cycles)
4h 45m
3 cycles
00:45
⚠ Fair (4 cycles)
6h 15m
4 cycles
23:15
✓ Good (5 cycles)
7h 45m
5 cycles
21:45
✓ Optimal (6 cycles)
9h 15m
6 cycles
💡 Sleep Tips
- • Aim for 5-6 complete cycles for best rest
- • One 90-minute cycle includes all sleep stages
- • 4 cycles is the minimum for feeling rested
- • Less than 3 cycles may cause grogginess
- • Keep your bedtime and wake time consistent
How the Sleep Cycle Calculator Works
A typical sleep cycle is 90 minutes long and runs through light sleep, deep sleep and REM in sequence. Waking at the end of a cycle feels far better than waking mid-cycle. The calculator offers either: 'I need to wake up at X, so what time should I go to bed?' or 'I am going to bed at X, so when should the alarm be set?'
It assumes 15 minutes to fall asleep and shows options for 3, 4, 5 and 6 cycles. The 5- and 6-cycle options are flagged as good and optimal; 4 cycles is fair (about 6 hours sleep) and 3 cycles is a minimum survival figure for a single short night, not a habit. Set a 7am wake-up and the tool will recommend going to bed at 9:45pm (6 cycles), 11:15pm (5 cycles), 12:45am (4 cycles) or 2:15am (3 cycles).
Recommended Sleep by Age
| Age Group | Recommended Hours | Cycles |
|---|---|---|
| Newborns (0-3 months) | 14-17 hours | Many short cycles, irregular |
| Infants (4-11 months) | 12-15 hours | 8-10 cycles |
| Toddlers (1-2 years) | 11-14 hours | 7-9 cycles |
| Preschool (3-5 years) | 10-13 hours | 7-8 cycles |
| School age (6-13 years) | 9-11 hours | 6-7 cycles |
| Teenagers (14-17 years) | 8-10 hours | 5-6 cycles |
| Adults (18-64 years) | 7-9 hours | 5-6 cycles |
| Older adults (65+) | 7-8 hours | 5 cycles |
Why Cycle-Based Wakeups Beat Hour-Counting
A 7-hour sleep that ends in light sleep usually feels better than an 8-hour sleep that ends mid-deep-sleep. That is why a 90-minute-cycle calculator can recommend a slightly shorter sleep on a busy night and still leave you feeling more rested than if you had grabbed an extra 20 minutes. Sleep researchers call the dazed feeling after a mid-cycle wake-up 'sleep inertia'; it can last 30-60 minutes and is what makes the snooze button feel so necessary.
Cycle length varies a bit between people (80-110 minutes is normal). If 90 minutes consistently feels off, try shifting your bedtime in 10-minute increments and tracking which start time produces the easiest mornings over a fortnight.
Frequently Asked Questions
Is 6 hours of sleep enough for an adult?
Six hours covers four cycles, which most adults can run on for a few days but not as a long-term habit. Adults need 7-9 hours (5-6 cycles) for cognitive function and immune health to stay where they should. Habitual short sleep is linked to reduced concentration, slower reaction times and weaker emotional regulation.
What if I cannot fall asleep within 15 minutes?
The tool builds in 15 minutes 'sleep latency' as a UK-typical average. If you usually take 30-45 minutes to drop off, add that gap onto the suggested bedtime. Sleep hygiene (no screens for the last 30 minutes, cool dark room, consistent wake-up time) brings most people closer to the 15-minute average within a fortnight.
Should I use this calculator for shift work?
It works for any wake-or-bed target, including night shifts. The bigger challenge for shift workers is consistency: rotating shifts disrupt the body clock no matter how cleanly you time the cycles. Aim for the same bedtime each working day, blackout curtains, and protect a 7-hour window.
How is this different from a sleep tracking app?
An app tracks what your sleep was actually like (movement, heart rate, sometimes audio). This calculator suggests when to set an alarm before the night begins. The two work well together: use this to plan, an app to verify, and adjust your typical cycle length if the data consistently disagrees with the 90-minute default.
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