Scientists claim to have found evidence that exercise may be physically addictive to some people.
The body may crave regular exercise
A study of mice found that when the exercise was denied to certain animals, brain scans revealed activity in areas normally linked to drug withdrawal.
University of Wisconsin researchers suggested that the same might be true in "extreme" individuals who tended to exercise more heavily.
Many experts believe that "exercise addiction" is not a physical condition.
However, there is plenty of anecdotal evidence from frequent exercisers who report feelings of craving when they miss their regular exercise session.
The study, published in the journal Behavioral Neuroscience, used a special breed of mice which, if given a running wheel to use, tend to use it for longer periods than ordinary mice.
At first, these mice, and ordinary laboratory mice, were given access to a wheel, and left to use it as much as they liked.
By and large, the "high activity" mice tended to cover more ground in the same amount of time on their wheels, perhaps three times as much on one of the days.
After six days, some of the mice from each group were prevented from getting to their wheels.
At the time of day when mice reach their "running peak", chemical measurements of brain activity for the all the mice were taken.
They found the mice denied their exercise showed higher levels of brain activity in 16 out of 25 brain regions. This was more pronounced in the "high activity" mice.
Dr Stephen Gammie, assistant professor of zoology at the university, said: "These mice have run for six days. They want to run, and they're ready to run, but they can't.
"Change in brain activity is an indication of their motivation to run."
Another researcher, Dr Justin Rhodes, said: "In the high-running mice, certain brain regions displayed extremely hgh levels of activity, more than normal.
"These were the same brain regions that become activated when you prevent rats from getting their daily fix of cocaine, morphine, alcohol or nicotine."
The researchers admitted that it was not yet clear whether the same phenomenon would hold true for humans.