Sleep isn't uniform — you move through roughly 90-minute cycles of light sleep, deep sleep, and REM sleep across the night. Waking up in the middle of a cycle (especially deep sleep) is what causes grogginess, even after enough total hours. Waking up between cycles, at a natural lighter-sleep point, is what makes mornings feel easier. That's why these times are spaced in 90-minute steps rather than just "8 hours before your alarm."

Recommended sleep amounts also shift with age, which is why the age group above changes which options are marked as ideal — a fixed rule for everyone isn't accurate.