That is a great question! The skin under our eyes is thinner than the skin on the rest of your face, so you can see blood vessels there more than anywhere else. When we don’t sleep enough, the blood vessels in our body dilate and you see that as a darker tint of the thin skin under your eyes.
Great question – I don’t know the answer either so have learn something new from Ioana’s answer! Now, what I would really like to know is how to get rid of them as it feels like I always have bags under my eyes at the moment… maybe that’s something new to research!?