Free tool · runs in your browser
Sleep Cycle Bedtime Calculator
Set the wake-up time and how long you take to fall asleep. The outputs are bedtimes, not exact sleep-onset times.
Cycle length is an average. The fall-asleep buffer is added before the cycles, so the bedtime shown is when you should be in bed, not when you should get in.
The arithmetic
Bedtime = wake time − (cycles × cycle length) − fall-asleep buffer. Times that cross midnight wrap back into the previous day, which is why the output stays a valid clock time.
What the result cannot know
It does not know your personal cycle length, whether you wake during the night, or how much sleep debt you carry. The five-cycle bedtime is a starting point; adjust after a week of consistent wake times.
Frequently asked questions
Why is six cycles earlier than five?
More cycles need more time, so the bedtime moves earlier. The six-cycle option is a long night, not the default.
What if my bedtime crosses midnight?
The calculator handles it and shows a normal clock time. A 7:00 AM wake time with five 90-minute cycles lands around 11:15 PM the night before.
Does screen use change the buffer?
It can lengthen the time it takes to fall asleep. If you scroll in bed, increase the buffer rather than pretending it does not happen.