"People have a limited pool of these valuable mental resources," Jaye Derrick, a scientist at the University at Buffalo's Research Institute on Addictions in New York, said in a statement last week. "When they use them on a task, they use up some of this limited resource. Therefore, they have less willpower and self-control for the next task."

"When you watch a favorite rerun, you typically don't have to use any effort to control what you are thinking, saying, or doing," Derrick said. "You are not exerting the mental energy required for self-control or willpower. At the same time, you are enjoying your 'interaction,' with the TV show's characters, and this activity restores your energy."

In one study conducted in Derrick's lab, a group of subjects kept a daily diary about their difficult tasks, media consumption and energy levels. When subjects had something difficult to do, they were more likely to watch a rerun on television, or watch a favorite movie or pick up a beloved book. Doing so, their energy levels were restored.

However, vegging out to any television program didn't have the same effect, the researchers found. "Just watching whatever is on television does not provide the same benefit," she said. "And perhaps surprisingly, watching a new episode of a favorite television show for the first time does not provide the same benefit."

The findings are published recently in the journal Social Psychological and Personality Science.