Question: Why do you get bags/shadows under the eyes when you are tired?

Susan Cartwright answered on 16 Jun 2015:

Eeek, what did I do to deserve this one? I’m not a physician or a biologist!!

However, I’ll do my best – but my best is pretty much an educated guess!

Patches of dark colour under the skin are usually caused by being able to see blood under the skin – for example, bruises occur when some of the fine blood vessels under the skin have been ruptured by a blow, so blood has leaked out and pooled under the skin. This is why you get a black eye if someone hits you in the face.

However, dark circles under the eyes aren’t that severe, so my guess is that the blood vessels that are always there are probably more visible than usual. I’m not sure what causes this, but I suspect that what happens is that fatigue causes your small blood vessels to swell up a bit, probably because of hormones released as a result of being tired. Therefore these blood vessels show up more clearly than they normally would. It’s most noticeable under the eyes because the skin around the eyes is very very thin, so it’s more transparent and the blood is more visible. The swollen blood vessels would also exert pressure on this thin skin, making it look puffy and producing the bags.