Check your Posture
Bad posture can easily lead to Repetitive Strain Injury (RSI) and eye strain especially if it at the same time you are working in poor light conditions.
RSI is a term given to a variety of related conditions which are caused by working a muscle to hard or at an angle which places strain and stress on the muscles.
If you are unsure about your own posture, try looking in a mirror side on. Also why not ask a friend or family member for their honest opinion on your shape and posture. Set yourself reminders to look at your own posture after working on a screen or playing video games for a set time, you will most likely notice your shape is different to your first thought.
Of course you should not overlook your desk or working environment, are the keyboards, mice and monitor at the correct height? Is your chair comfortable and provides plenty of support and is adjustable? If not then look into changing them, the costs if any will be minimal and allow you to work better and easier.
To help your posture you should look at your working area including keyboard, mouse and monitor ensuring these are all comfortable to use and within are at the wrong angle you should look at correcting all of these aspects promptly as these will help your posture and save you any discomfort in the future.