Brightness Differences Between Daytime and Evening

Hey everybody, I've just noticed something really logical but shocking today! My system is really bright during the daytime, but from around 7 and on it is so smooth and relaxing.. This is an AC problem I am guessing. It seems to be the only variable that changes, so that must be it. Another variable is ME, my mood during the day is different. I'm less relaxed and I've got more stuff running around in my brain. Just wanted to know what you think: Is it more ME or the AC that is contributing to the brightness during daytime hours. Let the games begin!