French kings at Versailles didn't sleep flat. They slept semi-upright, propped on a tall stack of bolsters and pillows.
Two reasons:
1. Medicine. Doctors of the 1600s and 1700s warned against lying flat, believing it let "stomach vapors" rise to the brain and cause muddled
I've been sleeping with the head of my bed raised for a while, and I'm surprised more people don't do it.
Inclined bed therapy (IBT) helps with acid reflux, sleep apnea, airways, puffy face, etc.
Basically, when you lie flat stomach acid can flow back up into your throat, and
