(Sheedy, Hayes, & Engle) found that reading with difficult environmental conditions such as upward gaze, glare, flickering light, or small text caused the participants to report symptoms of burning, irritation, and dryness.
While reading with internal problems like having to turn your eyes inward, astigmatism, and stress on the focusing lens caused the participants to report ache, strain, and headache.
Sheedy, J.E., Hayes, J., & Engle, J. (2003). Is all Asthenopia the Same? Optometry and Vision Science, 80 (11), 732-739.
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