Replying to Am I Really that tired?

SO recently I've found myself staring into space. Not like dozing off, but I lose focus of what I'm watching, and am suddenly focused on something farther away, or a point between me and what I was watching that doesn't exist anymore. The best name I can give it is eye strain. I'm 90% sure its either from being tired or from duck hunting, which is basically watching and trying to id these little specks in the sky. I'm wondering if anybody has had anything like this before, just cause I've never had this problem before. I can still look at any object and read anything, my vision is fine as long as I don't have to stare at something for a while.