Sleep Calculator
Ideal bedtimes and wake-up times based on 90-minute cycles
Sleep Cycle Calculator: When Should You Go to Bed?
Waking up tired often isn't about too little sleep — it's about waking mid-cycle.
Sleep runs in cycles of roughly 90 minutes: light sleep, then deep sleep, then REM, where most dreaming happens. If your alarm fires in the middle of deep sleep, you can feel groggy even after a full 8 hours — that's called sleep inertia. Waking at the end of a cycle, when sleep is lightest, leaves you far more refreshed. This calculator does exactly that math: enter your wake-up time and it lists the best times to go to bed, based on 90-minute cycles plus an average 15 minutes to fall asleep. Or tap "I'm going to bed now" to see the smartest times to set your alarm. Adults are generally advised to sleep 5-6 full cycles (7.5-9 hours) — the times marked "Best" hit that range.
Features
Bedtimes
Counts backwards from your wake-up time to the best bedtimes.
Alarm Times
Going to bed now? See when your alarm should ring.
90-Min Cycles
Based on the light-deep-REM sleep cycle model.
Fall-Asleep Buffer
An average 15-minute buffer to fall asleep is included.
Sleep Cycle Guide
How it's calculated
- One sleep cycle ≈ 90 minutes (light + deep + REM)
- Fall-asleep buffer: ~15 minutes
- Bedtime = wake-up time − 15 min − (cycles × 90 min)
- Wake at the END of a cycle to feel refreshed
How much sleep?
- 3 cycles = 4.5 hours (short — emergencies only)
- 4 cycles = 6 hours (lower limit)
- 5 cycles = 7.5 hours (recommended)
- 6 cycles = 9 hours (upper ideal range)
Frequently Asked Questions
What is a sleep cycle and why 90 minutes?
A sleep cycle is a repeating pattern of light sleep, deep sleep and REM. One full pass takes about 90 minutes on average and repeats 4-6 times per night. Individual cycles range from roughly 80 to 110 minutes.
Why do I wake up tired after 8 hours of sleep?
Your alarm is probably landing in the middle of deep sleep. Being pulled out of deep sleep causes grogginess known as sleep inertia. Aim for a wake-up time that lands at the end of a cycle rather than a round number of hours.
Why assume 15 minutes to fall asleep?
A healthy adult typically falls asleep in 10-20 minutes, so the tool uses 15 as a midpoint. If you fall asleep faster or slower, mentally shift the suggested times by a few minutes.
How much sleep do adults need?
Most sleep authorities recommend 7-9 hours per night, which corresponds to 5-6 full cycles. Teenagers need more; older adults often slightly less.
How long should a nap be?
Either keep it to 20-30 minutes — short enough to avoid deep sleep — or sleep a full 90-minute cycle. Naps of 45-60 minutes tend to end mid-deep-sleep and leave you groggy.
Is this medical advice?
No. This is a planning tool based on general sleep science. If you suspect chronic insomnia or sleep apnea, consult a doctor.