Other durations
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About a 12 hours alarm
12 hours is about 8 sleep cycles of 90 minutes. If you want to wake at the end of a cycle, the alarms below at 12 hours and 13 hours 30 minutes are the closest cycle boundaries.
The alarm is created as a one-time alarm at the exact clock time, so you can see it in the list above, rename it, or switch the sound. Keep this tab open with the volume up; on desktop it rings from a background tab, on a phone keep the page in the foreground and the screen stays on automatically.
Prefer a countdown?
If you want to watch the time tick down rather than set a clock time, use the 720 minute timer instead. Both ring with the same sounds.