Sleep happens in ~90-minute cycles. Waking between cycles (not mid-cycle) minimizes grogginess.
Pillow adds ~14 minutes for falling asleep, then calculates times that align with complete cycle boundaries.
3 cycles = 4.5h (minimum viable rest)
4 cycles = 6h (functional)
5 cycles = 7.5h (healthy)
6 cycles = 9h (full restoration)