That is an error in your brain's perception of things. When you use glasses for a while, your eyes get less used to straining and different muscles in your face

That is an error in your brain's perception of things. When you use glasses for a while, your eyes get less used to straining and different muscles in your face relax. Then, when you go without, you notice the strain. This same sort of thing happens when you switch prescriptions as well (not to mention that folks' eyes might degrade as part of normal aging). Realistically, these are the same sort of things that drive folks to the eye doctor because they are having headaches.