"Energy constrained" doesn't mean he lacks sugar, it means his brain isn't letting him do the work anymore because it is in the process of burning out.
While I cannot speak for anyone but myself, my performance on highly challenging tasks is capped at 4-5 hours per day. At that point, it is better for me to switch to lower hanging fruit.
If I am feeling especially inspired, sometimes I'll put in a 12+ hour day of hard work. Rarely, even two or more in a row. But inevitably, I will feel extra burned out in the subsequent days.