Many people have an appearance of dark circles on the lower eyelids, and they have many different causes.
Dark rings under the eyes are worsened by general fatigue, especially lack of sleep. The daily fluctuation is due to swelling of the skin, leading to a change in light diffusion, which looks like increased darkness of the skin.
For some people, all we can say is that their parents had dark circles under their eyes and therefore they do too. This trait can run in families, and is more pronounced in certain ethnic groups.
Sun exposure can also create dark circles under the eyes, by increasing the melanin content. The skin in this region can pigment more than the surrounding skin because it’s more sensitive.
Because the skin is thinnest under the eyes, the blood vessels here will be closer to the surface, meaning they look darker. As we age, our skin gets thinner and we lose collagen (the main structural protein in skin) and elastin (a highly elastic protein in connective tissue), which is why we get wrinkles. This often makes the blood vessels (which are dark in colour) under our eyes stand out more.
The tear trough (the depression below the eye) also deepens with age because of movement of fat under the eye forwards, creating shadowing below it.