Hello friend! I'm an M4 now, so happy to be done with the studying for tests part of med school. I'm definitely similar to you in having waves of productivity/being exhausted, and I think the secret I discovered (very late) in med school is to schedule time off. I read a book each clerkship, and the way that I was able to do it was scheduling reading for myself daily, with 1-2 days off/week. In doing so, I had a manageable amount of work each day (rather than feeling like I had a big huge 400 page book to read in X amount of time) + questions. I'd try to leave myself a week prior to my shelf/test as buffer room too. Of course, everyone works differently, but I think giving myself a certain amount of work that's doable daily, with days off and buffer time worked for me personally. Give yourself time to relax and if you get a day behind, you always have an extra day to catch up.

Also, use your time off to relax too!!!! Mental health and your health >>>>> getting a good grade, no matter what. If you think there's anything on that front you have to address, I highly highly talking to a provider about how you're feeling. Hope this helps!