Why do we get bags under our eyes if we don't get enough sleep?

See subject. I don’t even know what the physical description of those bags is, which would probably help figure out the cause.

Rhoda S. Narins, a clinical professor of dermatology at New York University Medical Center and the president of the American Society for Dermatologic Surgery, explains.

That article doesn’t explain why this is a physical response to not getting enough sleep, it just claims that it is one. I’ve wondered this too…

Swollen capillaries. Some mechanism of lack of sleep triggers inflammation - hence the warning about lack of sleep leading to heart disease, among other things. So the capillaries under the eyes in the bag area (IANAD, dunno what they’re called) swell, stretching the thin skin, causing mild bruising. We know that lack of sleep is bad for us, we’re starting to understand the many consequences of it - eyebags are a visible one of them.