Starting from the current time, this shows wake-up options that land on the end of a 90-minute sleep cycle. Pick the one closest to when you actually need to be up.
It takes the current time, adds the minutes it usually takes you to fall asleep (around 15 for many people), and treats that as the moment you drift off. From there it stacks 90-minute sleep cycles and shows where 4, 5, and 6 cycles land.
Waking near the end of a cycle tends to feel easier than waking in the middle of deep sleep. Choose whichever option is closest to your real alarm.
A few minutes either way is fine. Aiming for the nearest cycle, rather than an exact figure, is the version of this that is easy to keep using.
If your alarm is fixed and you want to work backward to a bedtime instead, the sleep calculator does that.
Open the sleep calculator