Easy to screw up your sleep cycle and hard to fix it. I use to do it all the time when I worked nights. I found that using NyQuil helped me fall asleep when I needed to and setting an alarm to wake me up when I needed to get up. After a few days of that it gets you back on track. Just try not to do it a lot or you will become dependent on it to sleep.
i screwed it up during finals week studying too late, and i'm feeling a weird anxiety (i think thats what it is) that won't let me sleep.