Having dry eyes has something to do with tear production. Tears are a vital part of the eyes as they keep them moist, clear, lubricated, and smooth. They contain water, oils, mucus, special proteins, and antibodies. Producing low quality tears or just simply secreting minimal amounts of it will dry your eyes out. Other causes include increased tear evaporation, old age, and having low levels of vitamin A and omega-3 fatty acids in the diet. 

Eyestrain is a condition reminding the patient that the eyes are tired and they need rest. It is caused by looking at the digital screen and reading books for a long time, driving long distances, fatigue, being exposed to bright light, fatigue, and stress.