Sleep
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Waking mid-cycle is why eight hours can feel worse than seven and a half. Sleep runs in ~90-minute cycles — time your alarm to a cycle boundary and mornings get kinder.
Why cycles beat raw hours
Sleep isn't uniform — it runs in cycles of roughly 90 minutes, each moving through light sleep, deep sleep and REM. Waking at a cycle boundary, from light sleep, feels clean; an alarm landing mid-deep-sleep produces that groggy, hit-by-a-bus feeling (sleep inertia) regardless of total hours.
How much sleep, though
Cycle timing optimizes the wake-up, not the amount — adults still need 7–9 hours (5–6 cycles) per major health guidance. Consistently taking the 4-cycle option builds real sleep debt. The 15-minute fall-asleep buffer is an average; adjust it to your reality.
The honest caveat
90 minutes is an average — real cycles run ~80–110 minutes and lengthen through the night, so treat these times as smart starting points and tune by feel over a week or two. Consistency (same times daily, even weekends) does more for sleep quality than any calculator. Persistent bad sleep despite good habits is worth a doctor's visit.
Sleep cycles FAQ
Sleep repeats in roughly 90-minute cycles through light, deep and REM stages. Waking at a cycle boundary — from light sleep — feels far better than an alarm interrupting deep sleep mid-cycle.
Five to six cycles (7.5–9 hours) matches the adult guidance of 7–9 hours. Four cycles works occasionally; three is an emergency floor, not a plan.
Eight hours often lands an alarm mid-cycle, in deep sleep, causing sleep inertia. Seven and a half hours is exactly five cycles — a boundary — which is why it can feel cleaner.
No — real cycles range roughly 80–110 minutes and stretch later in the night. Use these times as starting points and adjust a few days in either direction by how you feel.