Calculate optimal bedtimes and wake-up times based on 90-minute sleep cycles. Wake between cycles and feel energized.
Keep bedroom between 60-67°F (15-19°C). Cooler temperatures promote deeper sleep.
Avoid blue light 1 hour before bed. Use night mode or blue-light glasses.
Stop caffeine 6+ hours before bed. It has a half-life of ~5 hours.
Go to bed and wake up at the same time daily, even on weekends.
The Sleep Calculator finds the best times to go to bed or wake up based on 90-minute sleep cycles, so you wake at the end of a cycle feeling refreshed instead of groggy in the middle of deep sleep.
Sleep isn't uniform. You move through lighter and deeper stages in cycles of roughly 90 minutes, and waking at the end of one — in light sleep — generally feels far better than being pulled out of deep sleep partway through. That groggy, hit-by-a-truck feeling has a name, sleep inertia, and mistimed alarms are a common cause. Hence counting in whole cycles rather than round hours.
Both inputs are adjustable, and they should be. Ninety minutes is the commonly cited average; real cycles run roughly 70 to 120 minutes, vary between people, and vary across a single night — earlier cycles carry more deep sleep, later ones more REM. The 14-minute default for falling asleep is likewise an average worth replacing with your own honest estimate. If you routinely lie awake for 40 minutes, entering 40 will move every suggested bedtime by nearly three-quarters of an hour, which is the difference between the tool being useful and being noise.
This is arithmetic on an average, not a measurement of your sleep. It cannot see your actual sleep architecture, and cycle alignment is a distant second to total sleep — five well-timed cycles is 7.5 hours, which is fine, but three well-timed cycles is 4.5 hours, which is not fine no matter how neatly it lands.
It's also not a diagnostic tool. Persistent insomnia, loud snoring with daytime exhaustion, or sleepiness that doesn't improve when you sleep more are all worth raising with a doctor, and no calculator substitutes for that.
Sleep and body composition are the two things people most often try to fix in the same week — the BMI Calculator is the usual companion, with the same caveat that it's a screening number rather than a verdict. The Age Calculator is next door for the birthday countdown. The exact cycle arithmetic is written out on the methodology page.
Sleep happens in roughly 90-minute cycles. The calculator counts backward or forward in 90-minute increments, plus time to fall asleep, to suggest bed or wake times that land at the end of a cycle, when waking feels easiest.
Most adults need 7-9 hours, which is about 5-6 complete sleep cycles. The calculator targets full cycles so you wake refreshed rather than mid-cycle.
Waking in the middle of a deep-sleep stage causes grogginess, known as sleep inertia. Timing your alarm to the end of a cycle — what this tool calculates — helps you avoid it.
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