Driving home after a visit with a relative, you suddenly realize you have no specific memory of how you got there. Well, you've taken that trip so many times, you tell yourself, that you could just about do in your sleep. Tying a shoe later, you reflect again on how often you accomplish things while your conscious mind is barely paying attention. Of course, you're not wrong. We all have those moments.
At around three pounds, the gelatinlike tissue in your skull accounts for only a couple of percent of your total body mass, but it consumes a lot of energy—some 20 percent of the calories you eat every day. Conscious thought is “expensive” in energy terms. Is it any wonder the brain tends to shift its more costly processing tasks toward becoming more automated, “cheaper” routines?