Why do your eyes get crusty when you sleep?

Why does the area under your eyes turn dark when you don't get enough sleep?

• So, I usually can't sleep very well at night. I just start thinking of random things, and then I can't stop until like; 10:30 PM (I wake up at 6:05 AM every morning). So of course, the skin under my eyes turns purple. So, why does this happen to deprived sleepers anyway? I Please try to be as precise as possible. I tried my best to fit this into the right catagory.

• Answer:

  The eyelid skin is very thin. There are lots of blood vessels in the tissues under the lid. When you are lying down, this area swells a bit, and the blood vessels become more engorged. Seen through the skin, it looks bluish, just like a bruise looks blue when blood leaks under the skin.