Study tips and motivation?

I'm a junior in high school, and I get pretty decent marks from the 80-95 range. But this year, my marks have been really inconsistent, especially in math and my science courses. The only thing keeping my marks high are english and a few other electives. I know I have the potential to achieve higher scores, but I don't have any motivation at all! I don't even bother doing homework in math cl, and it's really putting a huge dent into my average. My irregular sleep schedule is a huge detriment to my academic problems and I don't know how to change. Does anybody have any study tips and/or ideas for motivation? Please help, I'm almost overwhelmed by my feelings of inadequacy.