Tired, so tired

I've been staying up really late the past couple of weeks and I'm sure it's just now taking it's toll. It's super weird though. It's really hard for me to wake up in the morning. Lately I've been waking up at about 7:20 to get to work in Provo at 8:00. Once I'm at work, I finish all of the morning stuff by about 9:15 then I crash hard. Currently I am having the hardest time staying awake. However, at 5:00pm I perk right up. I haven't been drinking caffeine so that isn't it. I think it's just that my body knows that I can sleep now, but it wont allow it. So, I'll go home and shower then go out for the night. I've lately been getting home around 11:30-12:00. But my brain doesn't fall asleep until around 2:00. Which is why I've been so tired lately. I keep telling myself that I'll get some sleep tonight, but it never works out. The really crazy thing is that once a week I'll fall asleep at about 9:00 and my body thinks that I've completely caught up on sleep. I just don't know what to do with myself. I am so tired!!!!!