The most important thing is absolutely getting sleep. When you don't sleep enough, your levels of cortisol production (cortisol is a hormone that makes you stressful) remain high, and you become worse at focusing and remembering information. I would prefer to get 8-9 hours of sleep on a test night than to study for an extra hour or two. It is that important.
Additionally, don't study for large amounts of time at once. Many different studies have shown that doing work in smaller chunks helps you focus better during those chunks. It's even better if you exercise during those breaks, as it has also been proven that you work more efficiently and with greater precision immediately after exercising than, say, in the sixth hour of a massive study cram.