system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do we toss and turn/constantly reposition ourselves during our sleep? What makes one position suddenly stop being comfortable even when we are not fully conscious?
gpt
Usually this is due to ischemia- lack of blood. When you're sitting in a position too long your body tells you to move, whether or not you realize it, to get the blood flowing back. If blood is robbed from an area of your body for too long, the tissue dies.

If you don't move, you get something called a pressure ulcer. Pressure ulcers have four stages and a last stage called unstageable. They are not pretty sights (do a google search). People get these because they cannot move when their body tells them to. Thus, the blood flow stops and the tissue in the area dies. In the hospital, we have turn schedules for our paralyzed patients to turn them every hour or two so their flesh doesn't die. 

When you're sleeping, your body is unconsciously moving to keep that blood flowing all while you rest peacefully.