I can't seem to grasp the difference between future perfect and imperfect. Can some please explain them?
it really depends on the verb. Spać does not have simple perfective alternative.
Będę spać = I will sleep /I will be sleeping - about the "activity"
prześpię (się)= it's about the time spent sleeping.
wyśpię się-it's about the feeling that your sleep is complete. /you have slept enough. (usually said about distant future when you have free time, including "wyśpię się po śmierci" :P