I personally have three tactics that I utilize when I am studying, each of which strengthen my understanding of the materials and simultaneously give me a break from reading.
1) As mentioned by others, I make sure to take notes as I read. This gives me a small break from the reading materials and forces me to re-word the material in a way that I intuitively understand. Sometimes simply taking a cryptic sentence or paragraph and rewording it in my own language goes a LONG way in understanding and retention. Also, by giving the reading a break and writing notes, I am switching gears and giving a different part of my brain some badly needed oxygen. :)
2) When my eyes start to glaze over, I'll often stop reading and looking over the material for concepts that I don't quite understand. I will then Google these concepts or related keywords in hopes of finding out more. This can be refreshing because you are stepping away from material that you might be getting bored with or that is not engaging you, and instead are stepping into new and fresh material that expands your understanding in different ways.
3) The last technique for me is the most important. I take a break from the reading and I pretend that a person I know well approaches me about what I am reading. This person begins to ask me questions about the material and is curious, but has no background on the material at all. I find myself telling this imaginary person all about the material but in the easiest laymen terms possible so that my friend can understand. I also explain things in such a way that the topic remains interesting. Analogies are really useful here. When you are able to explain a complicated paper in laymen terms that is easy to understand, then you know the material very well. After reading for an hour or two, I will often spend 15 minutes or sometimes much longer explaining what I've learned to my imaginary friend. I often find that this is one of the ONLY ways that I can retain knowledge of a new topic in long-term memory. I learned this early on in my career as I have a certain person in my life that actually models my imaginary inquisitor.
I think that you will find that the 3 tactics above are all taking breaks from the reading AND enhancing your understanding of the materials at the same time. I can sometimes study a single topic for hours on end if I remember to balance these three tactics as needed. I hope this helps!