"Any changes in hydration, whether

"Any changes in hydration, whether you’re dehydrated or you have salt retention because you have a big meal the night before, are going to reflect in that thin skin so easily compared to any other skin in the body," Ibrahim tells HuffPost. Specifically, being dehydrated will produce a more sunken look, while having too much salt (like if you consumed a really salty meal the night before) will lead to water retention and a puffy look.

How sleep in particular affects this water retention is still not completely known, Ibrahim says. However, he speculates that it could have something to do with the reason for why a person stayed up late the night before.

"Usually when people are up late, if they're doing something, they're usually drinking or partying or crying, and that is more of a contributor to why their eyes might be puffier," he says.

As for those dark under-eye circles, a number of factors could be at play. Ibrahim notes that it's the blood vessels under the skin that are actually responsible for the darkish hue, so when you're dehydrated, you can better see the color of the blood vessels (which appear blue under the skin). In addition, as people get older and begin to enter middle age, they will begin to lose volume around the eyes, producing what is called a "tear trough." This "tear trough" casts a shadow where the cheek and eyelid meet, which also contributes to the appearance of darkening. While these are general factors responsible for contributing to darkening around the eyes, Ibrahim notes that lack of sleep will only exacerbate these effects. So get your beauty sleep!