http://en.wikipedia.org/wiki/Hedonic_treadmill
Brickman, Coates, and Janoff-Bulman were among the first to investigate the hedonic treadmill in their 1978 study, “Lottery Winners and Accident Victims: Is Happiness Relative?”. Lottery winners and paraplegics were compared to a control group and as predicted, comparison (with past experiences and current communities) and habituation (to new circumstances) affected levels of happiness such that after the initial impact of the extremely positive or negative events, happiness levels typically went back to the average levels.
If things like winning the lottery or losing a limb tend to have short-term effects on happiness, then it's not surprising that a ~10% change in working hours has little effect. Especially if—as another poster mentioned—you just end up taking the work home.