Estimates only - not medical advice.
Sleep runs in roughly 90-minute cycles. Waking mid-cycle tends to feel rough; waking at the end of one tends to feel less so. The calculator adds about 15 minutes for the average time it takes to fall asleep, then counts backward (or forward) in complete cycles to give you times that land between cycles rather than in the middle of one.
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Roughly 90 minutes on average, though individual cycles range from 80 to 110 minutes. The calculator uses 90 as a standard estimate.
Most adults need 7 to 9 hours, according to the National Sleep Foundation. That is 5 to 6 full 90-minute cycles. Less than 7 hours on a regular basis is associated with impaired recovery and cognitive function.
That’s the average time it takes to fall asleep, so the schedule stays accurate.
Waking during deep sleep causes grogginess; aligning to cycles reduces it.
No - consistent timing, a dark room and limited screens matter too.