Sleep Calculator

Sleep is not uniform. Across a night you move through repeating cycles of light sleep, deep slow-wave sleep and REM, each taking roughly ninety minutes. How you feel on waking depends heavily on which stage you were in when the alarm went off. Being woken during deep sleep produces sleep inertia — that heavy, disoriented grogginess that can take half an hour to shake — whereas waking naturally at the end of a cycle, in light sleep, feels far cleaner.

That is what this calculator times. Rather than picking a round number of hours, it works in whole cycles and adds an allowance for the time it takes you to actually fall asleep, which is the step most people forget. Setting an alarm eight hours after getting into bed is not eight hours of sleep; if you take twenty minutes to drop off it is nearer seven and a half, and it may land you mid-cycle.

The ninety-minute figure is an average and should be treated as one. Individual cycles range from around seventy to one hundred and twenty minutes, and they lengthen as the night goes on, with more REM in later cycles. This means the alignment gets less precise the further out you project. It also means total sleep duration remains the more important variable — six aligned cycles beats five aligned cycles, and no amount of clever timing compensates for chronic short sleep.