It may feel like thinking happens instantaneously (for some of us), but there’s actually some lag time.
Scientists have approached this difficult question by timing how long it takes us to become consciously aware of sensory information. By some estimates, we can experience sensory stimuli that’s presented for as little as 50 milliseconds (about one-twentieth of a second). It is thought that our brains can, in fact, respond to information that’s much briefer than this, lasting less than a quarter of a millisecond.
In terms of sensing and then responding, a good measure is the sprinter reacting to the starting gun, which can be done in about 150 milliseconds. One limiting factor is how long it takes information to travel down our nerve pathways. In the 19th Century, Hermann von Helmholtz estimated this to be 35 metres per second, but we now know that some well-insulated nerves are faster, at up 120 metres per second.
Does concentrated thought burn calories?
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.