Sleep Cycle Calculator
Find the best time to sleep or wake up based on 90-minute sleep cycles. Waking between cycles helps you feel more rested. Allows 14 minutes to fall asleep. Everything in your browser.
Allows 14 minutes to fall asleep. Each sleep cycle lasts about 90 minutes, and waking up between cycles leaves you feeling more rested.
How it works
Through the night, sleep moves in cycles of roughly 90 minutes, rotating between light sleep, deep sleep and REM. When the alarm goes off mid-cycle, especially during deep sleep, you wake up groggy. Catch the end of a cycle instead and waking feels far more natural.
A healthy adult usually completes 4 to 6 full cycles a night, which works out to somewhere between 6 and 9 hours. The calculator adds 14 minutes for you to fall asleep and returns times that land at the end of a cycle. Everything is computed in your browser and nothing is sent to a server.
Sleep cycles of ~90 minutes
Human sleep runs in cycles of roughly 90 minutes. Each one moves through three NREM stages (N1 drowsy onset, N2 light sleep, N3 deep slow-wave sleep) before reaching a REM period, where most dreaming happens. Wake up at the tail end of a cycle, when sleep is shallow, and you feel far less of the grogginess known as sleep inertia than if the alarm pulls you out of N3. That is what apps like Sleep Cycle and smart alarms are trying to do. The calculator works backwards with bedtime = wake − (cycles · 90 min) − sleep latency, where sleep latency is taken as about 14 minutes. Example: to wake at 6:00 with 5 complete cycles (7h30m of sleep), go to bed at 22:06; with 4 cycles (6h00m), 23:36.
Clinical context and age
The American Academy of Sleep Medicine recommends 7–9 hours (5–6 cycles) for healthy adults, 8–10 hours for adolescents, and 7–8 hours for adults over 65, who tend to have slightly shorter cycles and lighter sleep. Sleep too little for too long and the deficit builds into a sleep debt that drags down memory, immunity and metabolic regulation. Treat the 90-minute figure as a ballpark. People vary anywhere from 80 to 120 minutes, the early cycles of the night carry more N3, and the later ones lean toward REM. Brazil's ANVISA approved melatonin as a food supplement under RDC 922/2024; if you use it, take it about 30–60 minutes before the planned bedtime.
The best time to sleep or wake up
Waking up groggy even after hours in bed has an explanation. Sleep arranges itself in cycles of about 90 minutes, and waking in the middle of one is what leaves you worn out. This calculator points out the best times to sleep or wake, slotted to the end of each cycle.
Tell it the time you need to wake and it shows when to lie down. Or say when you'll sleep and find the best moments for the alarm. The calculation also includes the 14 minutes we take on average to drift off, which makes the estimate more realistic than a bare number.
Tuning the time to your sleep's natural rhythm is a simple way to wake up more rested. Everything runs in the browser, and nothing is stored.
Frequently asked questions
Are 6 hours enough if I time the cycles?
Why do I sometimes wake before the alarm and feel fine?
Does the 90-minute figure apply to naps?
What if my schedule shifts every day?
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The results provided by this tool are for general informational and educational purposes only and do not constitute professional, financial, medical, legal, tax or accounting advice. Always confirm important decisions with a qualified professional and official sources.