I have always blamed the dark circles under my eyes on my children, since my dear ones wake me up during the night and very early in the morning. Thanks to the Mayo Clinic, my girls are off the hook.
It seems that those dark circles are most commonly caused by nasal congestion - who knew? When your nose is congested, veins that usually drain from your eyes into your nose become widened (dilated) and therefore darker.
Dark circles can also be caused by sleep - how ironic. Eyelids can swell while sleeping, since you are lying down, gravity can cause fluid to collect in your lower eyelids. This swelling may add to the appearance of shadows below your eyes. Unfortunately, there also seems to be a genetic component since they run in families.
So I guess using a neti pot might help more than my sinuses. It might also help with those darn dark circles.