Why You Wake Up Groggy

Sleep inertia — that disoriented, sluggish feeling upon waking — happens when your alarm interrupts deep sleep. Your brain needs time to shift from slow-wave activity back to full alertness.

The goal is to wake from lighter sleep, but the 90-minute cycle is only an average. Real cycles vary by person, by night, and across the same night. Use 6, 7.5, or 9 hours as rough planning math if helpful, but prioritize enough total sleep and a consistent schedule over a rigid formula.