It’s important to take some time off to recharge, even if it’s just a day or two. Sometimes my girlfriend and I will go to Disneyland, just to take a day off. If you feel completely exhausted while working, though, I’ve survived a long time on taking 20 minute naps. It just gives you a refresher. If the goal is to get the most quality time in, I’d rather set 20 minutes aside to get three hours worth of good work than just go through it all at 40%.