Idle Minds - why are our brains so active when doing ‘nothing’?

Kerri Smith does a review on the resting or default activity of our brains, what is happening when we are not focused on anything in particular, but letting our minds just rest or wander at random: After initial observations in the mid-1990s showed that the brain never really takes a break,

...studies of the resting state in its own right began to emerge. A team led by Marcus Raichle characterized activity in one such network as the brain's default mode — what they considered its baseline setting. During tasks, default-mode activity actually dropped, coming back online when the brain was no longer focusing so intensely.

The default-mode network has been joined by dozens of other flavors of resting-state network — some of which resemble the circuitry that contributes to attention, vision, hearing or movement. They seem very similar across study participants but are also dynamic, changing over time.

One idea is that the brain is running several models of the world in the background, ready for one of them to turn into reality.

Raichle favors the idea that activity in the resting state helps the brain to stay organized. The connections between neurons are continually shifting as people age and learn, but humans maintain a sense of self throughout the upheaval. Spontaneous activity might play a part in maintaining that continuity. “Connections between neurons turn over in minutes, hours, days and weeks,...The structure of the brain will be different tomorrow but we will still remember who we are.” …the brain replays and consolidates new memories at any chance it gets — even when awake.

…perhaps the activity is part of the reshaping process, tweaking connections while we idle. Several teams have reported changes in resting connectivity after language and memory tasks and motor learning… suggesting that the brain is not only thinking about supper coming up, but it's also processing the recent past and converting some of that into long-term memories. The network changes are specific to the tasks performed.