Just because that is true for you doesn't mean it is true in general. Like, imagine a surgeon -- would he/she spend 8 hours a week slacking? Or a radiologoist -- can he/she read the same number of slides in 32 than 40? Or a barrista at starbucks?
Also, if you worked 32 hours, would you still slack off? Is there a reason to expect that your slack/work ratio would change? You claim you'd be "in a better mood" -- but as humans we tend to adjust to the circumstances we are in. So I'm not sure that would be the case long term.