In side my chapter INSIDE THE BRAIN I detail recent work that has looked at the parts of the brain that are surprisingly active when we are in a resting state - day-dreaming - as opposed to being actively engaged in thought or upon some task. Key areas appear to be the medial parietal cortex, including a little-known area called the precuneous - and the medial prefrontal cortex. The latter area is known to become active when you are being reflective, particularly in the social sense of thinking about yourself in relation to others. This PHYSORG article details relevant work by Marcus Raichle on what he calls this default network. As he is reported:
"Raichle suspects that during these moments of errant thought, the brain is forming a set of mental rules about our world, particularly our social world, that help us navigate human interactions and quickly make sense of and react to information -- about a stranger's intentions, a child's next move, a choice before us -- without having to run a complex and conscious calculation of all our values, expectations and beliefs.
Raichle says such mental shortcuts are necessary because the brain cannot possibly take in all the detail available to our senses at any given moment. The default mode network, he proposes, keeps a template handy that lets us assume a lot about ourselves and the people and environment we interact with."
Interestingly these brain areas are very greedy - they have a high metabolic rate; they are disproportionately prone to the tangles of Alzheimers disease - which robs you of memory and a sense of self - while being overly protected in other ways - against stroke damage - by having two discreet sources of blood supply.