What causes humans to have dark circles under their eyes when they are not sleepy?

Herniated fat pads. The common cause of dark circles under the eye in the shadow created be bulging or herniated fat pads. This is and relatively easy surgical correction.
Many factors. Dark shadow can be due to skin pigmentation, superficial blood vessels, lower eyelid bags due to fat coming forward from the eye socket, and fat atrophy in the midface. See an oculoplastic surgeon to have this evaluated.