We've lost many institutions and much cultural knowledge since the Middle Ages. Catholic institutions were generally extremely well-optimized. Learning from them when possible is probably a good idea. I don't know how Catholics rested in the Middle Ages, but however they did, it was probably a reasonably good way of going about it.
Are you asking about Medieval Catholics in general or only those that lived according to a regula? I don't know the answer to either, but I think I know where to find it. Although, I do know that those living in a monastery usually spent the majority of their waking hours gardening (even more than praying or reading and copying).
So, it's well-known (or, at least, oft-thought) that you can't just work 16 hours a day; if you want to get stuff done, you need to rest from time to time. You have to take breaks.
Today, just now, I realized that I don't really know what the best way to rest is. If I want to rest, should I do something that's fun and interesting, like reading a fantasy novel? Should I do something that's boring, like building roads in Minecraft, so that work will seem comparatively interesting when I get back? (And besides, reading a fantasy novel is a bit of a challenge, at least for me, whereas building roads in Minecraft is trivial.) Physical activity probably helps, but how much? Do relaxing activities, like taking a warm shower, help more than just sitting there? How can I tell when it's time to take a break? How can I tell when it's time to get back to work?
(Suggestions for tags would be appreciated.)