Well, I was hoping another cadet would answer first and then I could build off of them but I'll give it a shot.
First: I want to preface what I say with every cadet is different and has different habits. Some go to bed before taps (2200), others go to bed much later...others rarely sleep and are caffeine dependent. Sometimes, its necessary other times I would just say its poor time management.
Personally, with the exception of 4/c year, I never do homework
during class. During class, either the instructor is teaching or leading a discussion or we get out early. The only time I ever did homework during class was during a few calculus classes first semester 4/c year.
Depending on the day, I usually do most of my homework at night after dinner. During the day, I use the free periods I have to get other tasks out of the way. This might include division work, laundry, running errands, working out, etc. Sometimes, I use a free period to relax and catch up on the news, facebook, a tv show, that sort of thing. To me, its not a waste of time if you plan on making up for it later.
As for sleep, I was pretty bad at getting to bed at a decent time until probably the end of 2/c year/beginning of this year. I actually work really well late at night so I got in the habit of staying up and going to bed at 12/1230/0100 every night which was fine until I started to realize that going to bed that late and then getting up at 0530 is miserable. Now, I do my best to be in bed between 2230 and 2300.
It also depends on the major. I'm a government major so I have more free periods because I don't have as many (or should I say zero) labs like the engineering or marine science majors do. My best friend here is a Naval Architecture major and she rarely slept more than 4/5 hours last semester. You just learn to adapt I guess.
Hope this helps a little bit, let me know if you have anymore questions!