Dark circles under eyes on a 4 year old?

MY 4 yera old sn has had these slightly dark puffy circles under his eyes. He's had this for years now and me gets pleanty of sleep and eats great. MY docrot noticed it the last time we took hom there for the flu and she asked if this was new and I said no after that she didn't seem to think it was anything to worry about, infact she didn't bring it up again. What could cause this? Neither my husband or myself have this problem under our eyes unless we are totally exhausted and out 6 year old has never had this either.