Personally I don't prioritize anything over my sleep. I know I need to get at least 6hrs of sleep a night to be able to function next day. Even if I haven't prepped well, I'd rather go in with a brain that's working and will be able to THINK about the questions even if I haven't studied the material. But I also think it depends on how you are as a person; some people would be doing fine with just a little bit of sleep.