Generally our brain burns 20% of the total energy needed on a daily basis in order to function normally. This means that it requires 400-500 calories a day, which varies according to how hard-working someone is. When we are awake we use a major part of our brain in order to function, use our muscles and our senses.
Experiments have shown that this is also relative to ones IQ. According to how clever someone is he burns more calories. This is because a clever person has to use his brain more and harder to solve a difficult problem.
At least now when I am going to be reading I will have in mind that I'm burning calories...!