Hi...  
I know the sleep issues bothers but I normally have 6 hours of sleep is fine.  Even when I was studying in University in UK(15 years ago), managing test and assignments are no fun.  Besides I get myself involved in the volunteer work, I used to be working during my free time(weekends included) where I will be on standby duty as a St. John Ambulance officer.  Its no fun, especially if its a 12 hour shift, I really crash at the end of the day.  I need a few days rest, then I'm back to normal.
Getting enough sleep is essential, I still need roughly around 6 hours sleep now.  If you are asking my hb, I have hb around 8-10.  When I was working, at times I was so busy with travelling and managing presentations, meetings and a family of 2 kids.  Its even harder then to get a good night's sleep.  I am ok, if I have 4 hours sleep once in a while but I will crash at the end of the week if I persistently have only 4-5 hours sleep in a day, meaning I need to rest for the next 1-2 weeks to get back the boost.
The best for you is to nap.  I find this very useful.  If I feel like crashing, I need to nap, I mean 10-15 minutes light sleep.  I normally set my alarm just to make sure I would wake up at the end of the nap.  The nap will get you going for at least a few hours.  Long hours of sleep during the day is not only unproductive, it makes you even more tired at the end of the supposedly to be a restful period.  The most useful sleep is during the night, if you are able to sleep through.  I find that very essential to have good night's rest.  
I hope this little piece of advice will help you.