Depression Support Group
Depression is a real and debilitating condition that is often misunderstood by family and friends. Its meaning can range from a prolonged period of sadness to an actual mental illness with specific symptoms. Find and share experiences with others who are going through the same struggles.

I would talk to an endocrinologist. Get labs done, find out if it's MEDICAL. If there are no medical reasons, then it could be major depression.
If it just happens once in a while i wouldnt worry much.. but if it happens several times a week to several times a month.. start digging.
Possibilities:
major depression
diabetes
Anemia or hemochromotosis (severely low or high iron)
anxiety disorders
thyroid dysfunction
there are just sooo many things.. the best place to start is medical then work from there. but please find help because you dont want to sleep your life away. there are a lot of things to experience out there.