Why do I feel so tired?

I'm 14 years old in year 10. I eat healthily and do enough excercise but I am just so exausted all the time. We live quite far away from school an my mum has this weird thing were she makes us Hoover the whole house in the morning so that takes some time so I have to get up at 5:45am every morning for school. I realize this is earlier than the times my friends get up for school although I go to bed much earlier. I am just so tired though. My joints all hurt and I'm constantly walking slouched and collapsing at my desk after getting to each lesson. I'm always told off for sleeping in cl and now I'm behind on homework because after my day at school I'm just too exausted to do it so I just fall asleep. I get an awful lot of homework to an coursework and now I have 4 exams coming up in the next month so now I have to revise for them and do coursework etc. Having a social life us completely put of the question cause I have like no time at all. My friends think it's hilarious that I'm constantly sleeping. but now I'm so tired I'm actually getting really emotional. I cry at stupid things like I wasn't hungry an I didn't want my dinner an then my mum told me I had to eat it so I just burst out crying. It's just stupid :( xxx please help me!!! I've been drinking lots of green tea lately aswell to wake myself up a bit but it doesn't work x