I sleep ~4 hours a night during the week. I find that if I sleep more (~6-8), I am not tired again by the time the remaining hours have passed and it is time for me to sleep again. For instance, when I sleep 6 hours, I am not ready to sleep again 18 hours later. This becomes a real problem after two nights or so, at which point I have a lot of trouble falling asleep.

I do tend to sleep more on the weekends, but I often end up essentially forcing myself to sleep come Sunday night.