I now do about 2-3 hours a day on the computer and have done for the last 4 years or so. During this time my eyesight has deteriorated to the point that I am barely passing my Class one eyesight test and I'm convinced it's due to this "close" work. I'm desperate to avoid glasses becoming a condition of my licence because I can't think of a less natural circumstance than parading around with bits of glass propped in front of your eyes.
I realise that age has its effects but I'm sure that modern living has a much more immediate impact. I've started to try some of the natural "eye training" methods such as those preached by Dr. Bates and to date, I've made a little progress. Hope this helps - we were all hunter gatherers 20 thousand years ago and we're designed to focus at a 20 yard hunting range not an email inbox!