Yes, but not as much as you might hope. Under deep anaesthesia, your brain uses about six or seven calories an hour, just to keep the brain cells ticking along. Experiments with rats have shown that just regaining consciousness increases the energy demands of the brain by 50 per cent, even if the body is kept anaesthetised. So even if you spend all day fast asleep, your brain will still burn at least 240 calories. The total daily consumption of your brain is 400-500 calories (20 per cent of your total energy requirements) so that’s just 160-260 calories for your waking brain activity. And most of this is concerned with the largely automatic process of controlling your muscles and processing sensory input. There have been some experiments to show that abstract problem solving does raise the brain’s metabolic requirements, and the higher your IQ, the more extra energy you can expend in this way. But it’s unlikely to amount to more than five calories an hour, either way. So you can’t Sudoku yourself slim.