Sleep cycles
Wake time = bedtime + (N × 90 min) + ~15 min to fall asleep
Sleep runs in ~90-minute cycles; waking at the end of a cycle (rather than mid-cycle) leaves you feeling far more refreshed. Most adults need 5–6 cycles (7.5–9 hours). We add ~15 minutes for the time it takes to actually fall asleep.
Sleep in complete cycles
Sleep runs in cycles of roughly 90 minutes. Waking at the end of a cycle usually feels far better than being jolted awake in the middle of deep sleep. Tell the calculator when you must wake up and it counts back in 90-minute cycles (plus ~15 minutes to fall asleep) to suggest ideal bedtimes — or do the reverse from a bedtime. The starred options (5–6 cycles) are the sweet spot for most adults.