Personally, in my experience, I have found it to be because of the constant thinking and amount of concentration required. Our brains use a disproportionately

Personally, in my experience, I have found it to be because of the constant thinking and amount of concentration required. Our brains use a disproportionately large amount of our bodies resources and I believe this leads to large amounts of stress when done too much. I'm a big believer that programmers should not code more than 4 hours per day so that our minds have enough to rest and recover properly.