We have a limited supply of willpower?

Researchers asked 16 people to perform self-control tasks while being monitored by an fMRI scanner. During the first session, people were assigned to either a demanding mental task or easier task. Two weeks later, they swapped tasks.

The results: Brain scans from the first session showed promising activity in both the participants’ anterior cingulate cortex (ACC)–an area that deals with decision-making–and dorsolateral prefrontal cortex (DLPFC), an area that helps manage self-control.

But after the second session, those who were exposed to the demanding task first showed less activity in their DFPFC. Simply put, “if you exert a significant amount of self-control at one time, you’ll have a hard time exerting it later,” says lead study author William Hedgcock, Ph.D., a neuroscientist at the University of Iowa.