Difficulty concentrating wit mental task

This could be due to Computer visions syndrome (CVS). Majority of them who spend 3+ hours on computers daily are said to suffer from CVS to some degree. It results from concentrating on a computer monitor for extended lengths of time without interruption, and symptoms are dry, irritated, and strained eyes, headaches, double vision, polyopia, difficulty refocusing the eyes, and, in extreme cases, nausea, dizziness, and a general not well feeling. Some useful tips: Avoid low lit room, there should be adequate lighting. Massage your eyebrows gently using both your palms Blink your eyes to relax the muscles Stare away from monitor atleast every 15 to 20 minutes,  walk around and come back after couple of minutes. Set the fonts size suitable to your eye vision. Close eyes periodically for some time when you are compelled to work on computers for long hours. Consult an Opthamologist who can suggest eyedrops. Don't go for self medication. Eyes are very sensitive organ and not worth experimenting with any random commercial drops.