A thin layer of tears covers the surface of the eye. This later dries out when exposed to the air. When we blink, our eyelids spread a new tear layer.
Looking straight ahead or up at a monitor exposes more of our eye's surface than when we look downward. A letter to the New England Journal of Medicine reported a study on tear evaporation (Tsubota 1993).
Researchers found that tears evaporate at a faster rate when we gaze straight ahead at a monitor than when we look downward in a natural reading position.
The authors found that subjects blinked less and had an increased rate of tear evaporation when using a VDT than under relaxed conditions.
Our eyelids partially close when we look downward. This reduces the surface of the eyeball exposed to the atmosphere. It also helps counteract the effects of reduced blinking.
The researchers suggest that computer users lower the monitor and tilt the screen upward.
Their report confirms a 1987 unpublished study by the late Bruce Rupp (1987) of IBM. Rupp found that when subjects looked downward at a 45-degree angle, they exposed over 40 percent less of the eyeball's surface than when they looked straight ahead.
In addition to reducing the exposed surface, a Downward Gaze™ angle also increases the volume of tears our eyes produce (Sotoyama et al. 1997). So not only does a Downward Gaze™ angle cause the tears to evaporate more slowly, there are more of them produced.