1 hour nap every 4 hours awake. Been giving this advice to those who cram like you do my entire college career. Of course, I always suggest NOT cramming and getting a good night's rest every night, but people live differently so what can I do.
Best to sleep right after you eat something, so it will ease you into sleeping. It's not healthy, but then again what you're doing is already destructive. Try to drink juices or caffine free soda, water would be better. Stay away from coffee, tea, milk-tea, ice cream shakes, or anything else overly caffinated or creamy.
Stay away from medications, especially caffine pills or sleeping pills. They'll just make you loose focus and you won't retain anything.
Study in a comfortable sofa or chair, try not to study on your bed. You might doze off or start wandering, since we usually think when we lay down in bed at night and stare at the ceiling, so that might happen to you if you accidently drift off for a moment.