Why do I get so tired?

No matter how much I sleep, around two or three in the afternoon I hit a wall and I am ridiculously tired. It’s not like a little tired, I can’t think and caffeine doesn’t do anything. I do have depression and anxiety and I take an antidepressant. I also have migraines so I don’t know if it’s just part of a migraine. Is it something I should be worried about?