“Mental energy” is a pretty common phrase, and it refers to the ability to start and sustain work on tasks which require mental effort. Eating a candy bar can provide a temporary boost to mental energy, but it’s not an effective everyday approach.

There are many physical and psychological issues which diminish people’s mental energy. ADHD and depression being the most obvious.

Fortunately for many of those conditions there are also ways to increase the amount of mental energy you have available.

And yet this advice is not necessarily wrong. We spend a significant fraction of the calories we eat operating our brain. I have personally measured a significant increase in my ability after a good lunch compared to just an hour or two prior.

"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.

Yes, this is my intended meaning. I am a PhD student working on my thesis. After a certain number of hours of doing mathematics or algorithm design, I have to switch to easier tasks like reading papers in a domain I'm familiar with, or documenting code.

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.