system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why Do We Get Dark Circles Under Our Eyes

I've been told its because of lack of sleep, but that seems like a falsity.  What's the real reason?
gpt
First we need to understand the *physics* before getting to the biological reason for it. The reason the area under your eyes are prone to **darkening** (be it dark circles or a punch to the face) is the skin under your eyes being thinner(0.5mm) compared to the rest of your body(average 2mm). Add to that the high density of blood vessels and capillaries in that region. And you have light reflecting off the back of the blood vessels sitting under that patch of incredibly thin skin, giving it a darkish appearance.

Now the biological reasons for the darkness increasing are not one but many; with genetics playing the more important role than others. Some of the causes are:

1. *Old age* : As people age their skin loses elasticity and the ability to regenerate and as a result becomes thinner. This is the reason older people predominantly have very prominent dark circles.
2. Preorbital Hyperpigmentation is a genetic condition in which more melanin is produced in the skin below your eyes.
3. Another curable illness causes damaged blood vessels below your eyes to leak small amounts of blood.
4. **Fatigue**: Yes fatigue can indeed cause dark circles but they are by no means the predominant factor. When the body is tired, production of the chemical **cortisol** is dramatically increased to help give you the energy you need to stay awake. Among many other things, cortisol actually increases the volume of the blood in your body, which causes the blood vessels (including the ones below your eyes) to engorge to accommodate it. 

tl;dr: Yes fatigue can cause dark circles but genetics and ageing are  the most likely cause of developing them.

edit: improved the answer a little bit.