Sleep Calculator — Sleep Cycle Bedtime & Wake-up Times

Sleep is organized in cycles of roughly 90 minutes, each passing through light sleep, deep sleep and REM. Feeling rested in the morning depends not only on how long you slept but on where in a cycle your alarm fires: waking at a cycle boundary from light sleep is easy, while an alarm that lands in deep sleep produces the heavy, disoriented feeling known as sleep inertia. This calculator plans your night around those boundaries.

It works in both directions. Tell it when you need to wake up, and it counts backwards to give you a list of bedtimes — each labeled with the number of complete cycles and hours of sleep it buys you. Or tell it when you are going to bed, and it lists the smart alarm times going forward. In both modes it automatically adds 15 minutes, the average time a healthy adult takes to fall asleep.

Recommendations adapt to age using commonly cited sleep-duration norms: school-age children need about 9–11 hours, teenagers 8–10, adults 7–9, and people over 65 around 7–8. The options that fit your selected age group are highlighted, so you can aim for the realistic best choice rather than the theoretical maximum.

The calculator is free, needs no account and runs entirely in your browser — nothing about your schedule is sent anywhere. It is a planning aid, not a medical device: if you regularly sleep enough hours and still wake up exhausted, that is worth discussing with a doctor.