I'm not sure about the genetics of it (none of my family members seem to agree with me), but I also perform computer tasks far better in a dim light. A friend of me, who is also a night-owl, has the theory that when we get exposed to daylight, our cavemen heritage's natural reaction is to feel like going hunting/food gathering. While I don't buy the argument verbatim, I feel more like doing outdoor/physical activities on sunny days.