Sleep isn't uniform — it moves through repeating cycles of roughly 90 minutes, each passing
through light sleep, deep sleep, and REM (when most dreaming happens).
Waking up in the middle of a cycle — especially during deep sleep — causes sleep inertia:
that groggy, heavy feeling that can linger for a while. Waking between cycles, when sleep is
lightest, makes getting up much easier.
This calculator counts backward or forward in 90-minute steps, plus the time it takes you to fall asleep
(about 15 minutes on average — adjustable in settings), so your alarm lands at the end of a
cycle instead of the middle.
How many cycles you need depends on age. The National Sleep Foundation recommends 7–9 hours
(about 5–6 cycles) for adults, more for teens and children, slightly less for seniors. Cards are colored by how
close each option is to your recommended range.
90 minutes is an average — real cycles range from about 70 to 120 minutes and vary
night to night. Treat the suggestions as good targets, not exact science.