Most commonly it is the result of fatigue in the ciliary body, but there are many other causes of eye strain. One of the functions of the ciliary body is to change the shape of the eye’s lens during accommodation, so it is in constant use when you are focusing on something. If you focus on one thing too long, fatigue can set in and eye strain can occur.
An overuse syndrome, eye strain is a simple issue that can cause far reaching and chronic problems. It can be a problem for children and their education. It can affect your performance at work. It can get worse as you age.