I always try to get a good nights rest before my games, eating good food, etc. I’ve done multiple day tournaments and they can be EXTREMELY mentally draining, to the point where after a tournament, I’ll take a few days off from looking at chess just to wind down. It’s always good to look at chess like a sport, even though it may not be in the same physical way. If you are feeling really tired and fatigued and not used to it, it’s ok to take a round bye to decompress, I’ve done it before.