The human brain uses about 200 kilocalories per day, which equates to approximately 10 watts of power. For comparison's sake, that's about 10 percent of the power that it takes to run a 100-watt light bulb. So the cartoon image of a light bulb over your head when a great thought occurs isn't too far off the mark. Your brain generates as much energy as a small light bulb even when you're sleeping.