The first three hours of sleep

Growth hormone secretion in adults is heavily concentrated in slow wave sleep, and the largest pulse of the night typically lands in the first slow wave period, roughly the first hour or two after you fall asleep. A majority of daily secretion happens in that window.

Growth hormone is one of the signals involved in tissue repair, including skin. This is the mechanism behind the phrase everyone uses and nobody explains.

The part that gets missed: the pulse is tied to the first deep sleep period, not to the clock. Going to bed at two and sleeping eight hours does not simply move the window later without cost, because slow wave sleep competes with circadian timing.

Nothing we do in a treatment room outperforms a consistent bedtime. We say that to guests fairly often, usually while they are lying down with their eyes closed, which is a reasonable place to hear it.