Re: when you like what you do...
I used to have a job I loved, I often spent long hours doing it. It being IT related I used IT to work out how efficient I was being. I found out I could work the odd week of more than 60 hours and productivity would improve a bit. After a couple of weeks of 60hours or more productivity would plummet below the 37.5 hr normal week and take a week or two of normal work to recover. This was largely due to the error rate increasing.
After some visits to the US where 7am-7pm and at least some hours over the weekend was 'compulsory' and I realised no work was being done because there was no urgency I stopped doing long hours unless absolutely necessary. Pissed a lot of people off but no-one could get the work done as fast as me.
I've since worked for several bosses who worked long hours and got fuck all done.
I've since found out some German states fine companies who make their employees work too long and they have higher productivity.
All the science points to working longer gets less done as the errors accumulate. It looks like 30-40 hrs is pretty much optimal.