Yeah. Everyone is different. I don't have a spouse, kids or dog. I like it when I'm working on a problem and the world goes away for 10-12 hours at a time. I can get a lot of work done in a week where I work 60-70 hours. I'm sure many people can't and on average, 40 hours may be optimal but there is no way that 40 hours is optimal for me when I'm doing something interesting.

Having said that, I've also been burnt out working for startups. You can do 50-60 hours a week for many months but at some point you need a break. I've done it for 18 months but after that, I was pretty useless for several months.

One way I take a break is to work as a software contractor (which I'm doing now.) This pays well and is almost always a 40-hour per week job. It also tends to be pretty boring but it gives me a chance to recharge.

BTW, to relate this back to the topic, I just turned 50 and hope to continue writing software for at least another 20 years.