system
Given the following post, respond as if you're commenting on Reddit:
human
eli5: How are eyebags formed?
gpt
The dark circles or bags that form under your eyes when you stay up too late are caused by the tiny muscles in that area fatiguing, and fluid building up in that area. Because the skin is so thin and delicate in that area you can really see it. Most people think bags come from not getting enough sleep, but it's actually from straining the eyes beyond the point when your body wants to rest or go to sleep.