Why do I get dark rings under my eyes?
The skin under the eyes is very thin, so it will show any color changes in the blood vessels underneath. These changes can occur when you stand upright for extended periods of time without lying down for sleep. Other causes include histamine release (mild allergies) and dehydration. The medical term for a bag under ones eyelid is a Festoon.
Let's discuss the causes:
The blood supply from that skin drains into the internal jugular vein. That drainage is a lot better when you're lying down than when you're standing up - so the blood tends to pool there. This is one factor that will give you dark half-circles under the eyes.
You have huge numbers of Mast Cells in the skin under the eyes. These Mast Cells will release histamine (sometimes by themselves, and sometimes when you rub them), which will cause swelling under the eyes - and darkness. You can rub over-the-counter anti-histamine skin cream on your lower eyelids can stop histamine release and prevent bags.
When you get dehydrated (which sometimes happens when you get tired), the skin under the eyes gets dark. And finally, not a reason but an observation. Eyes in animals are an important signaling area. When you see another human with dark areas under their eyes, you know they are tired.
Excess Skin from Aging
The natural aging process leads decrease skin elasticity to excess skin beneath the eyes. Depending on genetic factors, as well as environmental factors such as smoking, you may notice these changes sooner or later in life. A plastic surgeon can remove some skin and underlying fat to improve cosmesis. Surgeons can also remove excess skin and fat that is causing puffy, drooping eyelids. Remember that studies show even the best plastic surgery only removes about 3 years of aging from one's face.