And I'm a co-founder. People really do overwork themselves (myself included). The most important thing is to make sure you spend your time well. If that means sitting around for a week doing as little as possible, in between busting ass on the important stuff when it comes along, by all means it's more productive to chill out for a bit.
Part of the reason I took so long away when my daughter was born, was to set an example in the company that we can be more productive if we treat ourselves well.