eye strain 
noun [mass noun]
Fatigue of the eyes, such as that caused by reading or looking at a computer screen for too long: avoid eye strain and have your sight checked regularly...
- Why do some computer work-stations confer eye strain and muscle fatigue?
- Side effects include eye strain, headache, fatigue, irritability and inability to sleep.
- A recent study has found that concentrating on difficult off-screen objects, such as the faces of loved ones, causes eye strain in computer scientists.