The brain runs on glucose. This is why shots of "energy gel" every 45min or so during a long run keep one energetic; the legs use glucose too, and though the legs can keep going long after glucose runs out the central nervous system gets all sloppy. Brain work really does make one feel hungry. Problem is the brain doesn't actually burn a lot of calories, it just burns glucose. A 25cal shot of a teaspoon of honey would solve the problem. Or walking around for 10 minutes and letting the body pull blood sugar out of stores. Or working out regularly, since the body stores glycogen in muscle tissue and the more muscle you have the more glycogen you've got to convert to glucose. Or not having diabetes since the body has problems regulating blood sugar. Basically the more you are a shitlord and the more you're into lifting or cardio, the less likely you are to suffer cravings for sugar during mental work. Seems to check out- when I was a ha  7ffa  m I always felt a sugar craving at work, now that I have 130 pounds less fat and 30 pounds more massels I'm like "nope, I'm good all day."