Trying to perform two or three demanding activities at once is not only stressful and inefficient, but it also drains the brain of energy.

It turns out we only think we are performing multiple tasks at the same time. In fact, neuroscientists have discovered that we are actually rapidly switching back and forth between tasks—a process that winds up reducing our ability to focus attention on any one subject and burning up precious glucose needed to fuel the brain.

The result may be fatigue and “brain fog” that makes it harder to think clearly or recall information. Research suggests that even such apparently mundane tasks as shopping can be adversely affected by multitasking. An article in the Journal of Retailing found that shoppers asked to buy three snacks totaling less than 500 calories were more likely to exceed the caloric limit if they were multitasking by talking on their cellphones. The researchers theorized that the stress caused by trying to accomplish two tasks at once was responsible for the negative impact on distracted shoppers’ decision-making.

This article originally appeared in Mind, Mood & Memory, a publication of the Department of Psychiatry at Massachusetts General Hospital, dedicated to maintaining mental fitness for middle age and beyond.

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