When learning a new task, the brain is very active. Neurons are firing, and synapses are being connected at a breakneck pace. Once a task is learned though? That's when the brain takes a little break. When a chore is being performed that is routine, such as the same drive home every day, or the same way of making a meal, the brain is, in a sense, on autopilot. The task is so familiar that the brain doesn't have to work that hard.
To demonstrate this, researchers at Cambridge University in the UK conducted a study using MRI scans of brain activity. It's known that the area of the brain called the default mode network (DMN) is active when a person is daydreaming or thinking about a past event. The volunteers in the study were told to match a card to one of four cards shown to them. To "win" the study participants had to figure out if the match was supposed to be by number, color or suit. When learning the game, the dorsal attention network was more active. Once the task was learned, activity decreased in that area of the brain and increased in the DMN. The information gained from this study will hopefully help those with neurological conditions like PTSD or OCD who often have automatic thought patterns that disrupt their ability to live a healthy life.