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Sleep Calculator
Best bedtime and wake time based on 90-minute sleep cycles. No signup — your inputs stay in your browser.
Step By Step
How to Use This Calculator
4 steps
- Choose 'When to wake up?' if you want to plan your bedtime, or 'When to go to bed?' to find your ideal wake time.
- Enter your wake time or bedtime using the time picker.
- The calculator shows four options — 4, 5, 6, or 7 sleep cycles — so you can pick what fits your schedule.
- Aim for at least 5 cycles (7.5 hours). Six cycles (9 hours) is marked as recommended.
Worked Example
Example: Bedtime 10:30 PM, wake up mode
Use this sample to sanity-check your inputs and understand what the final result represents.
- 1Fall asleep: 10:30 PM + 15 min = 10:45 PM
- 25 cycles: 10:45 PM + 7h 30m = 6:15 AM
- 36 cycles: 10:45 PM + 9h 00m = 7:45 AM
Final Result
Aim to wake at 6:15 AM or 7:45 AM to complete full sleep cycles.
Methodology
90-minute sleep cycle + 15-minute sleep onset
This section explains the calculation logic, assumptions, and source material used to make the result more trustworthy and easier to verify.
Wake time = bedtime + 15 min + (cycles × 90 min). Bedtime = wake time − 15 min − (cycles × 90 min). The 90-minute average cycle duration is from Carskadon & Dement (2011), Principles and Practice of Sleep Medicine. 15-minute sleep onset from Ohayon et al. (2004), Sleep, 27(7).
Frequently Asked Questions
Why does the calculator use 90-minute cycles?+
A full sleep cycle runs through light sleep (NREM 1 and 2), deep slow-wave sleep (NREM 3), and REM sleep — and takes roughly 90 minutes on average. Waking at the end of a complete cycle, when you're in the lightest sleep stage, generally feels easier than waking mid-cycle during deep sleep. The 90-minute figure comes from polysomnography research (Carskadon & Dement, 2011).
Why does it add 15 minutes?+
Most adults take 10–20 minutes to fall asleep after getting into bed — a measure called sleep onset latency. The calculator adds 15 minutes to account for this so the wake time reflects actual sleep time, not just time in bed. If you fall asleep faster or slower than average, mentally adjust the result.
Is 6 cycles (9 hours) really recommended?+
The National Sleep Foundation recommends 7–9 hours for adults 18–64. Six 90-minute cycles = 9 hours of sleep time, placing it at the upper end of that range. Five cycles (7.5 hours) is closer to the midpoint and works well for most people. The calculator marks 6 cycles as 'recommended' because it comfortably fits the NSF window — pick what your schedule allows within the 7–9 hour range.
Does everyone need the same amount of sleep?+
No. The NSF ranges account for individual variation — some adults feel well-rested at 7 hours, others need 9. Age, genetics, health conditions, stress, and activity level all play a role. The rare 'short sleeper' gene exists but affects less than 3% of the population. If you're regularly sleeping under 7 hours and feel fine, it's worth discussing with a doctor rather than assuming you're in that group.
What if I can't fall asleep within 15 minutes regularly?+
Taking more than 20–30 minutes to fall asleep regularly is called sleep onset insomnia. Common causes include screen light before bed (blue light suppresses melatonin), caffeine too late in the day, an irregular sleep schedule, anxiety, or underlying sleep disorders. Cognitive Behavioural Therapy for Insomnia (CBT-I) is the most evidence-based treatment — more effective long-term than sleep medication for most people.