For the first time in human history, that question has an engineering answer. It doesn't scream. It hums.
If you are in deep sleep, the patch remains silent. It waits. It knows patience. When your brain naturally flips into N1 or N2, the vibration begins. Because your sensory gating is already lowered in light sleep, the brain perceives the vibration not as an intrusion, but as a continuation of your own biological rhythm.