Stressed out because of exams. Help?

I am a student of XI Th std and have recently joined a coaching institute for physics,chemistry and biology. I had always been an A1 student till class X but somehow I feel that I am deteriorating in class XI. I have to attend my coaching class for 4 hours and that too, for 5 days a week. I get so tired by the end of the day that I simply fall asleep. Overcome by exhaustion, I go on sleeping till 11 AM in the morning. My sleep cycle has been disturbed to a large extent; and to make matters worse tomorrow is my first Unit Test in the coaching class. I feel so stressed out and don't know what to do. There seems to be no end to biology and no matter how hard I try, I always end up memorising a mere 4-5 pages everyday. I have been doing this for the past 2-3 weeks but alas, 25 pages of botany are still left and I've never had time to touch zoology. I feel as though I'll either fail in the exams, or pass with very little percentage, or that my record of coming 1st time will be broken (not by coming 2nd or 3rd but somewhere beyond the top 50) :((