Plan your sleep around 90-minute cycles. Wake up at the end of a cycle, not in the middle of deep sleep.
A complete sleep cycle averages 90 minutes and includes light sleep, deep sleep, and REM. Waking at the end of a cycle (rather than mid-cycle) leaves you alert; waking during deep sleep causes grogginess (sleep inertia). Adults typically need 5–6 cycles per night (7.5–9 hours). This calculator adds 14 minutes for falling asleep.