system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 Why do people and some animals involuntarily stretch when we’re tired or waking up? Is it the same reason as to why we yawn?
gpt
It’s natural almost to the point of being impulse. It’s your bodies way of preparing for movement. If you’re waking up, it helps push blood flow to limbs in case you need to go chase gazelle. 

When you’re wanting to sleep, this is your bodies way of trying to keep you alert and ready for movement, like if an animal is about to hunt you in the dark.