Time your sleep in full 90-minute cycles instead of a flat number of hours.
How to Use the Sleep Calculator
Choose "I want to wake up at..." and enter a wake-up time to see four suggested bedtimes, or choose "I'm going to bed now" and enter (or use the current) time to see four suggested wake-up times.
Both modes count backward or forward in whole 90-minute sleep cycles, then add 15 minutes on top for the time it typically takes to actually fall asleep.
Wake-up at: bedtime = wake time − (cycles × 90 min) − 15 min
Going to bed: wake time = bedtime + 15 min + (cycles × 90 min)Six cycles is nine hours of sleep plus that 15-minute buffer — go to bed 9 hours and 15 minutes before you need to be up. The calculator lists three, four, five, and six cycles so there's a range to choose from rather than one fixed number.
Why Cycle Multiples, Not Round Hours?
Sleep runs in stages, and waking up mid-cycle — especially out of deep sleep — tends to produce the groggy, disoriented feeling known as sleep inertia. Waking up between cycles, at a naturally lighter point, tends to feel easier, which is the entire premise behind timing sleep in multiples of a cycle length instead of just picking "8 hours" and calling it done.
If six cycles puts your bedtime earlier than you'd like, three or four cycles is a genuine option too — less total sleep, but timed to end at a lighter point rather than cut off mid-cycle at random.
The 90-minute figure itself is a population average, not a personal one — real cycles commonly run anywhere from about 70 to 120 minutes, and shift from person to person and night to night. Nothing short of an actual sleep study (polysomnography) could measure your real number, so treat the four times here as a starting point to experiment with, not a guarantee.