From the Mayo Clinic:
Almost everyone gets dark circles under their eyes from time to time. Dark circles under eyes usually are temporary and not a medical concern.
You may be surprised to learn that fatigue usually isn't the cause of dark circles. Instead, the most common cause is nasal congestion. When your nose is congested, veins that usually drain from your eyes into your nose become widened (dilated) and darker.
Another cause of undereye circles is eyelid...