I really love my job, and when I am working on 'the good parts' of it, I find it very hard to stop at 40 hours (ok, honestly, usually it's a little bit more like 45). I do force myself to stop, in large part because I have a family at home who deserve to know me. But I've also found that when I not working, filling my time with some other things than work really does enrich my life, and I feel that my performance at work does increase, or at least is more consistent because of that.

In any case, I see very clearly, also in people around me, that loving your job and working extremely long days are not the same thing.