system
Given the following post, respond as if you're commenting on Reddit:
human
What causes the circles under your eyes when you're tired and what causes them to go away when you sleep?

My troll brain is keeping me awake wondering this.
gpt
Adenosine. It is an endogenous (originating within the body) neurochemical that has many and diverse effects including vascular dilation and muscle relaxation. It also gives you that sleepy feeling and induces the stereotypical behavior of preparation for sleep: retiring to a quiet and dark place and lying still.

Adenosine, overall, inhibits the release of excitatory neurotransmitter chemicals including norepinephrine. Norepinephrine causes the layer of muscle surrounding blood vessels to contract. Adenosine-caused inhibition of norepinephrine release allows blood vessels to dilate. Thus the dark color under the eyes. The shortage of norepinephrine also allows the muscles around and behind the eyes to relax, allowing the eyeballs to sink into the skull.

As the day progresses the concentraion of adenosine in your bloodstream gradually increases, until there's enough to give you that sleepy feeling. As you sleep the concentration of adenosine gradually decreases, until you awake.

Anything that increases adenosine concentration will tend to make you feel lethargic, sleepy, or tired. For example: trauma (tissue damage) releases large amounts of adenosine from ruptured cells. That's why a wound or injury can make you feel tired. In extreme cases enough adenosine is released to cause the condition called shock.