Find someplace comfortable that has little to no distractions. For me that was the beach. I would sit in my beach chair and when my mind or eyes needed a break I could look up take in a deep breath or two and then get back to it.
Another trick is listening to classical music while you study. And yes I mean Bach and Beethoven, not the Beatles and the Beach Boys. The driving beats of modern music can actually make you tired. (Which is why you shouldn't listen to it nonstop on road trips either.)