Tukuk-OS😴 Sleep-Cycle Calculator
ℹ️ What this tool does & how it works
Sleep runs in cycles of roughly 90 minutes; waking inside deep sleep is the grogginess people blame on “lacking sleep”. This page counts back from your alarm in whole cycles (6 × 90 = 9 h, 5 × 90 = 7.5 h, 4 × 90 = 6 h) and adds about 15 minutes to fall asleep. The honest caveat is printed with every answer: real cycle length varies 70–120 minutes between people and nights, so this is a good model, not a measurement — and it makes no medical claim.
Where the 90 minutes comes from
Measured polysomnography cycles — light sleep, deep sleep, REM — average close to 90 minutes in adults, first appearing in sleep research of the 1970s. The average is the point: your personal mean may be 80 or 100, and the model cannot know it without data. If the predicted wake-times consistently feel wrong by the same amount, that offset is your personal cycle length — nudge the math by it rather than trusting 90 forever.