> Scientists who spent 25 hours in the workplace were no more productive than those who spent five. Anecdotally this is a concept I've run into in many areas of life, the most overt one being school, I suppose because it's so easily quantified. You can do a moderately acceptable amount of work on any given assignment and get a C+, work hard for a B/A or work your hands to the bone for an A+, but at the end of the day…
I'm not sure I buy the "you get diminishing returns for working harder" in general --- yes, you can absolutely burn out, but you could also just do amazing things if you work really hard. Sometimes there are diminishing returns (like the A vs A+ in school), and sometimes there aren't.
The most important thing is that progress can be made even if you want to work at a more leisurely pace on difficult problems. And, of course, that's far less likely to burn you out.