Vicious cyrcle

My life habits have greatly changed these weeks. I used to go to bed at about 10 or 11 o’clock when I lived at home. But after living in the dorm, I always go to bed after 12 o’clock. At first, I feel everything is good as before. I can get up early and get good vigor as before. But a few weeks later, I get tired after I get up in the morning and often feel sleepy and even nod off in class. I try to return to the normal life before but there are many reasons that make me replay the same life day after day. I have to find a solution quickly because I don’t want to be tired but concentrate in class.