It is often claimed that periorbital dark circles are caused by tiredness or working too hard or even just staying up late. While this can be true, the truth is that your genes play a huge role here.

Simply put, dark circles are a result of the thin layer of skin below your eyes showing the blood vessels and the blood they contain more clearly than anywhere else on your body. (For reference, this skin around your eyelids, called periorbital skin, is on average about 0.5 mm thick compared to an average of about 2 mm thick on most of the rest of your body.) That is the mean reason you have to choose the best under eye cream for the dark circle.

Now, the reason veins often look blue isn't because the blood inside them is blue; it's because your skin/subcutaneous tissue only lets blue/violet wavelengths of light pass through it. As a result, only blue light is reflected back and the veins look, well, bluish. (Veins often won't appear blue if a person has darker or lighter skin. Rather, the veins tend to appear green or brown. On the other hand, people with extremely light skin, will typically have veins that show up as dark purple or dark red, more closely resembling the actual color of the blood running through the veins.)

It's the exact same concept with the skin below your eyes. Those dark circles are (usually) just the result of light being reflected back off of the blood vessels sitting just below the surface of that incredibly thin patch of skin. This is the same reason facial bruises are more prominent below or around the eyes; the thin skin just shows the blood from the ruptured blood vessels a little bit more clearly.

Again, as we age, our skin loses its elasticity and ability to regenerate and as a result, it becomes thinner. This is why more often than not, elderly people will have rather prominent periorbital dark circles regardless of how much they sleep. As with those who are genetically predisposed to having thinner skin below the eyes, it's important to start early use the best under eye cream for dark circles.

Another thing that can cause dark circles beneath the eyes is something known as periorbital hyper-pigmentation, which is basically a condition that results in more melanin being produced by the skin below the eyes, resulting in it appearing to be a darker color. This is mostly a condition that affects (or is at least more noticeable in) darker skinned people. 

Another problem that goes hand-in-hand with periorbital dark circles is periorbital puffiness, aka bags below your eyes. In young folk, this is almost always caused by fluid buildup below the eyes, either due to illness, allergies or simply excessive salt consumption which can result in the body retaining more fluid than usual. This can place increased pressure on the skin and blood vessels around the eyes which can force blood vessels closer to the surface of the skin, making dark circles appear more prominent. Eye puffiness can also simply be a result of old age which you can perfectly solve by choose the best under eye cream for dark circles.

Now we explain if those dark circles be caused by fatigue.

Well sometimes yes, but It's not like lack of sleep makes your skin thinner.

It's how the body acts when it's low on energy. 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. As we've already mentioned, dark circles are mostly caused by us seeing our blood vessels / blood through our skin, so it stands to reason that when those blood vessels are engorged, they'd be easier to see, even in people who may have been blessed with thicker skin below their eyes.

So, whether by passing on their genes to you or for not letting you sleep in after a gaming all-nighter when you're young, it's all your parent's fault. Something that is/was even more reckless of them as cortisol promotes fat accumulation. So your lack of sleep is likely contributing to more than just under-eye cosmetic issues.