I think you're 100% right. It's impossible (for me anyways) to be productive 7 days a week for a lot of weeks in a row. I started recently taking Sundays off from working altogether and spent the day only doing stuff like going to church, relaxing with friends, reading, exercising, and maybe a couple other things. I think it has helped me in lots of ways, from being more efficient and effective at work the other days to also enjoying the work more.