Sleep Calculator
Find the best bedtime or wake-up time based on 90-minute sleep cycles.
Sleep happens in cycles of about 90 minutes — waking up between cycles, rather than in the middle of one, tends to feel more refreshing.
Go to bed at one of these times
Assumes about 14 minutes to fall asleep — adjust the time yourself if you know you take longer.
About this calculator
How This Sleep Calculator Works
Enter either a wake-up time or a bedtime, and the calculator works backward or forward through complete 90-minute sleep cycles to suggest times that align with a natural point to wake up, plus a 14-minute buffer to fall asleep.
Worked Example
To wake up at 7:00 AM, going to bed at 9:46 PM allows for 6 full cycles (9 hours), 11:16 PM allows for 5 cycles (7.5 hours), and 12:46 AM allows for 4 cycles (6 hours).
Why Sleep Cycles Matter
Sleep moves through repeating cycles of roughly 90 minutes, alternating between lighter and deeper stages, ending in REM sleep. Waking up in the middle of a deep-sleep stage tends to feel groggy and disorienting (called sleep inertia), even after a longer total sleep time — while waking up at the boundary between cycles, when sleep is naturally lighter, tends to feel more refreshing even with fewer total hours. This is why this calculator works in 90-minute increments rather than just subtracting a flat 8 hours.
A Note on Individual Variation
90 minutes is an average — actual cycle length varies by person and by night, roughly 70-120 minutes. Treat these suggested times as a starting point to experiment with rather than an exact science, and prioritize consistent sleep and wake times overall, which has a bigger effect on sleep quality than hitting an exact cycle boundary.
Worked example
To wake up at 7:00 AM, going to bed at 9:46 PM allows for 6 full sleep cycles (9 hours) plus about 14 minutes to fall asleep.
Frequently asked questions
Why 90-minute cycles?
A full sleep cycle — moving through light sleep, deep sleep, and REM — takes roughly 90 minutes on average. Waking up at the end of a cycle rather than in the middle of one (especially deep sleep) tends to feel less groggy.
Is 90 minutes exact for everyone?
No — individual cycle length varies, typically 70 to 120 minutes. This calculator uses the commonly cited 90-minute average as a starting point, not a personalized measurement.
Why does it add 14 minutes?
That's a typical estimate for how long it takes an average person to fall asleep after getting into bed. Adjust your actual bedtime earlier if you know you take longer to fall asleep.