You know doing mentally taxing actions can make you feel tired, but how much energy does our brain actually use? Ewan McNay, an assistant professor of psychology and behavioral neuroscience at the University of Albany has studied how the brain uses energy to perform work. This data has showed that the brain, unlike other parts of the body, runs only on the sugar glucose, and strenuous cognitive activities require more glucose than simple ones. McNay says, “You will in fact burn more energy during an intense cognitive task than you would vegging out watching Oprah or whatever.” However, in the context of the average person’s over energy expenditure, he says the difference in calorie burn from one mental task to another is a tiny amount. In a typical day, a person only uses about 320 calories just to think, and there’s really no way to think yourself slim, as changes in brain activity and energy use during a tough mental task are extremely small.