Hey Spidey,
One of my friends had this problem. He was having to go to sleep for an hour or so after school because he was so tired when he got home. He went to the doctors and guess what the problem was? He was eating a lot of chocolate during the day which was causing this effect, even though he was physically fit and otherwise healthy. He stopped the chocolate and the problem was solved! Not sure if yours will be that simple but as others have mentioned, look at what you are eating and see if anything stands out. Hope you get it sorted soon mate.
Best wishes,
Voltar