[unholiness]

Yeah, I hear a lot of people saying essentially "40 hours is the most anyone can be productive for in a week". My more realistic rule of thumb is that people can tolerate maybe 20-30 hours of doing things they don't intrinsically care about.

So, if you give a shit about 20% of your job, and your job tolerates you doing personal stuff (slash miscellaneous browsing) for a few hours a week, then sure, 40 hours can be sustainable. On the other hand, if this job is your passion, work is stimulating, and you've got things you feel positive ownership over, then having 60-80 hour work weeks isn't necessarily unhealthy (so long as you're aware if and when those drives start to dwindle).

The other upshot of this theory is it dismantles argument that side projects hurt employees' productivity. The extra time spent pursuing your own passions simply isn't draining in the same way implementing others' ideas is. If anything, it adds motivation to the day-to-day stuff, knowing you'll learn skills at work that help the side project you really care about.