The muscles around the eyes are designed to point both eyes in the same direction. Some people have a condition called a Phoria. Under normal conditions the eyes are straight but if one eye is covered the eye behind the cover tends to drift out of alignment. The eye can drift in, out, up or down. A large Phoria can sometimes cause eye strain and headaches and in some cases, double vision, particularly when you’re tired. Reading can also be difficult and tiring. Phorias can usually be corrected with eye exercises or glasses.