The findings explain why Facebook or Twitter are such popular diversions for people who feel like taking a break.
"The brain has a major system that seems predisposed to get us ready to be social in our spare moments," said senior study author Matthew Lieberman.
"The social nature of our brain is biologically-based," Lieberman, a University of California Los Angeles (UCLA) professor of psychology, added.
The research helped resolve a nearly 20-year-old mystery.
Neuroscientists have known since the 1990s that the brain includes a network of regions that seems to be most active during periods of rest.
The UCLA research shows that during quiet moments, the brain is preparing to focus on the minds of other people - or to "see the world through a social lens".
"When I want to take a break from work, the brain network that comes on is the same network we use when we are looking through our Facebook timeline and seeing what our friends are up to," Lieberman said.
The study involved 21 participants viewing 40 photographs in three sets, with their brain activity recorded using functional magnetic resonance imaging (fMRI).
Most of the photos showed people performing actions in a social setting and expressing a certain emotion. In one set of 40 photographs, images were paired with captions that reflected the person's mental state - 'He is feeling bored' or 'She is expressing self-doubt', for example.
The second set of photos had identical images, but with captions that merely described what the person was doing - 'He is resting his head' or 'She is looking to her side.'
And a third set of images depicted a number accompanied by a simple mathematical equation - for example, '10: 18-8'.
The researchers found that the regions of the brain that were active while viewing the first set of images (emotion) were also active during periods of rest.
Those areas were not as active when looking at the two other sets.
Also, one part of the brain, the dorsomedial prefrontal cortex (which turns on when we dream and when we think about other people), was more active during rest, before participants were asked to look at photos.
Interestingly, the dorsomedial prefrontal cortex was less active when participants viewed the other two set of photos, even if they were the same photos but with non-emotional captions.
In general, the findings could also explain why many people want to socialise with others in the workplace.
The study was published in the Journal of Cognitive Neuroscience.
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