A new study has revealed that it all comes down to water retention when people end up with swollen and red eyes and droopier eyelids, basically looking like hell, after a bad night's sleep.
Dr. Sherrif F. Ibrahim, from the University of Rochester Medical Center, told the Huffington Post that since the upper and lower eyelids have the thinnest skin on the body, any changes in hydration are going to reflect in that skin so easily compared to any other skin in the body.
How sleep in particular affects this water retention is still not completely known, Ibrahim said, but he speculates that it could have something to do with the reason for why a person stayed up late the night before, like if they are drinking or partying or crying.
Ibrahim added that dark under-eye circles can be caused by many reasons, but it's the dehydrated blood vessels under the skin that are actually responsible for the darkish hue.