In general, it seems normal to get tired at night, doing homework, if you are up and going at 5:45 in the morning. You lead a healthy lifestyle and that includes getting between 7-9 hours of sleep a night. If you are constantly on the go, with little rest, your body is going to react to that. Staying up all night is not healthy nor normal, so I don't think that should worry you. However, if you are instantly dosing off without even knowing it is coming, then that could be a medical condition. If you are seriously worried about this then I think it would be in your best interest to see a doctor - if anything to ease your mind.