Our eyes are one of our most valuable possessions, yet we often take them for granted, even abuse them.
With winter days shorter and grayer, we spend more time indoors under artificial light. Reading or studying in this light takes more "eye energy" than in the natural light of day because artificial lighting doesn't include the full light spectrum. Reading in a dimly lit room is even worse; it can cause eyestrain. So give your eyes a break—turn on an extra light or two if you're going to be reading inside.
Another simple thing you can do is rest your eyes periodically. Indoors, most things you look at are fairly close to you, and your eyes work harder to focus close up. On the other hand, gazing at a distance relaxes the eyes. So an easy way to rest your eyes is to simply look out a window at the sky or at the horizon.