system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does the little stretch and shake that we do before or after sleeping feel good? What does it do the body?
gpt
When you sleep, your muscles lose tone and fluid tends to pool along your back. Stretching helps to massage fluid gently back into the normal position. It also activates your parasympathetic nervous system and increases blood flow to your muscles. It's thought that stretching may also release endorphins that help to reduce pain and enhance your mood.