Free Online Sleep Calculator
Find the best bedtimes and wake times around sleep cycles.
Sleep cycles average about 90 minutes. Waking at the end of a cycle usually feels easier than waking in the middle of deep sleep.
These times assume you fall asleep after the delay above and wake at the end of a cycle. Consistency, caffeine, light exposure, and room temperature affect sleep quality as much as timing.
About the Sleep Calculator
Sleep is organised into cycles of roughly 90 minutes, moving through light sleep, deep sleep, and REM. Waking in the middle of deep sleep often produces grogginess, while waking at the end of a cycle usually feels clearer. This calculator works backwards from a wake-up time, or forwards from a bedtime, to suggest times that land near the end of a sleep cycle.
How the times are calculated
In wake-up mode the calculator subtracts a falling-asleep allowance and then the chosen number of full cycles from your alarm time. In bedtime mode it adds the falling-asleep allowance and the cycle length to your bedtime. The 15-minute default is a common average for sleep latency, but you can adjust it to match how long you actually take to drift off. The 90-minute cycle length is also adjustable, because individual cycles vary.
Four cycles give about six hours, five give about seven and a half hours, and six give about nine hours. For most adults, seven to nine hours is the recommended range, which makes five or six cycles the usual target. Shorter sleep is not automatically better just because it fits neatly into a cycle.
Frequently asked questions
Should I aim for exactly 90-minute cycles?
No. Cycles vary between people and across the night, and the first cycles tend to contain more deep sleep. Treat the suggested times as helpful approximations rather than exact targets.
Why do I still feel tired after seven and a half hours?
Duration is only one factor. Sleep quality, consistency, stress, alcohol, caffeine, and underlying health conditions all matter. If fatigue persists, speak with a healthcare professional.
Is it better to wake up earlier or later than the suggestion?
Most people do better with a consistent schedule and enough total sleep. If you must choose, avoid cutting below seven hours on a regular basis rather than chasing a perfect cycle boundary.