If you're used to sleeping 5 hours a night, then it's okay. Try getting up at the same time every morning... it's even better if you can go to sleep around the same time each night. Get your body into a routine. You'll get used to it, and will be able to function normally without feeling like shit (most of the time).
However, more sleep does make a big impact. Use sport as an example. Functioning on 5 hours of sleep may be enough for classes, and you may feel it's fine for sport as well. However, you may find that if you get into a routine of more sleep (e.g. 8 hours), then you may be able to push yourself that much further and perform better.
A lot of it is psychological. Get into a routine, and stick to it (most of the time).