I wonder too. It would be really nice, and I should try it. But, I do remember Jason Fried telling me that their four day week didn't work out because it effectively became a 3.5 day week, and that simply was not enough time to run the business. 3.5 days is 28 hours in a 40 hour week. And the 37signals employees seem to do a lot of deliberate practice on their own time.