It is important that your concentration levels are high while studying. There is a famoouse figure of 8 concentration exercise. It is very simple. Whenever you feel you are losing concentration while studying start drawing the figure of 8 in the air (or a blackboard if you have one) with your right hand, then the left hand and then both the hands. Continue making the figure smaller and smaller till you make them really small. This exercise can be done by closing your eyes and imagining that you are drawing the figure of 8. This exercise helps to activate both the right and left visual fields and hence makes the brain more focused.

I ll start waving my hands in 8s just in case people in the PhD room don't think I am weird enough....Thanks!

It could look weird if you are doing this in the middle of the class, but you can pass on this exercise to your phD friends too and then actually see if it helps. I do it on paper and i end up feeling more focused as my mind closes to other thoughts and i get engrossed in this. So when all the distraction doors are shut for a while, you can concentrate better.