One of the most healthy things you can do for your brain and your body is to get up out of that chair you seem to be glued to and take a walk. Even if it is one block you will get your circulation going. Seriously it is very important that your circulation is not being cut off from sitting too long. Other very important health issues are your eyes, your wrists, and your lower-back.
You need to rest your eyes. You can either look out the window or somewhere in the distance for a few minutes so as to train your eyes that they can see further than you keyboard. You may also prefer a ‘cat nap’ – just to close your eyes for a few minutes and rest. Eye drops for ‘dry eye’ and other uncomfortable eye conditions (burning, red, etc.) may help you as long as you don’t overdo it and get sufficient rest. Your monitor should be eye-level – again – not up and not down but straight ahead.
With the wrists the issue is repetitive motion and the position that you hold them. Your hands should be level with your keyboard – not up and not down – straight ahead. The same goes for your mouse which can become painful especially if you lean forward to use it – it will be a pain in the neck and/or shoulder literally.
For your lower back if you have something as simple as a phone book that you can keep your feet on it will help immensely to lessen the stress your lower back is feeling from too much sitting. There is nothing better than getting up out of the chair hourly or so to just move around and circulate. If it is not convenient to take a real walk, just walk around your home office.
However you should definitely make time for at least a few real walks. Especially after you have eaten a meal there is nothing more unhealthy than sitting down all cramped up. So really try to take at least a short walk after meals. Maybe even do the dishes would at least be standing.
You may laugh at ergonomics and all this and cop a cavalier attitude about your health. Chances are someday you will see what is really happening and hopefully it won’t hurt too much and you will still be able to sit at your computer to work.