Why is it that you can't sleep when you know that you have to get up early (much earlier than you usually do)

I am supposed to be getting up at the ass crack of dawn (5 in the a.m. and it’s now 11:30 p.m. my time) and I can’t seem to settle down. I’ve had it before where I couldn’t settle down, and stayed up most of the night when knowing that I had to get up really early.