As a comp sci major, I noticed that before crunch time/finals I would try to sleep, but while in bed my mind would race trying to solve non-existant coding problems or algorithims for my programs that I didn't even need to write. Its hard to explain, but it sounds very similar to what your describing here. What helped for me was to not work on school for at least an hour before going to bed, and reading a book instead. Give that a try!