Dark circles under the eyes

From New Scientist

... rings are often a sign of being chronically worn out, stressed and run-down - tiredness rather than sleepiness. Tired people don't just need more sleep, but a better and more agreeable lifestyle. This is easier said than done, of course. They may even find that they have difficulty sleeping despite their tiredness, but resorting to sleeping tablets is not the answer.

As for the anatomy of these rings and whether they are due to blood pooling, skin-thinning through dehydration or something else, no one really knows.