"Every time we slip effortlessly into a daydream, a distinct pattern of brain areas is activated, which is known as the default network. Studies show that this network is most engaged when people are performing tasks that require little conscious attention, such as routine driving on the highway or reading a tedious text. Although such mental trances are often seen as a sign of lethargy - we are staring haplessly into space - the cortex is actually very active during this default state, as numerous brain regions interact. Instead of responding to the outside world, the brain starts to contemplate its internal landscape. This is when new and creative connections are made between seemingly unrelated ideas.
"When you don't use a muscle, that muscle really isn't doing much of anything," says Dr. Marcus Raichle, a neurologist and radiologist at Washington University who was one of the first scientists to locate the default network in the brain. "But when your brain is supposedly doing nothing and daydreaming, it's really doing a tremendous amount. We call it the 'resting state,' but the brain isn't resting at all."