The eye is coated by a thin layer of fluid called the tear film. It evaporates while the eyes are open and is replemished with each blink.
During prolonged visual concentration, the rate of blinking reduces so that the tear film has more time to evaporate between blinks. This is exaggerated in dry air and draughts. Tear production diminishes with age and so, it is common to experience dry eye symptoms when reading, watching TV or using a computer.