You can’t concentrate at peak efficiency continuously, so be sure to take breaks. When you need a break though, take a real break. I used to meditate or nap on the grass between classes in order to recharge myself. I’d use my wristwatch alarm to signal when it was time to get up and go again. Those breaks were very restorative, and I could go to the next class and work full out once again. I never worked flat out all day long. I worked in waves between total concentration and total relaxation, cycling many times per day.
How long were these breaks?