Last updated 23 August 2026
Why cycles, not just hours
Sleep moves through light sleep, deep sleep and REM in roughly 90-minute loops. Waking up in the middle of one — especially during deep sleep — tends to feel groggier than waking between cycles, even with the same total hours. Timing sleep to complete cycles, rather than a round number of hours, is what this calculator is aligning to.
Common questions
Why 90 minutes?
A full sleep cycle — moving through light sleep, deep sleep and REM — takes roughly 90 minutes on average. Waking up in the middle of a cycle, during deep sleep, tends to feel groggier than waking between cycles, even with the same total time asleep.
Why add 15 minutes?
That's a typical estimate for how long it takes to actually fall asleep after getting into bed, so the cycle count lines up with time asleep rather than time in bed.
Is 90 minutes exact for everyone?
No — it varies by person and by night, roughly 70 to 120 minutes. This is a useful average to plan around, not a precise measurement of your own cycles, which really only a sleep study can show.