The University of Wisconsin-Madison just published a research study that you may find interesting. Their research with mice indicates that "exercise is addictive" due to the increased release of dopamine in the brain when the mice were kept from running.
Article:
Bred at UW, running rodents may yield clues to addictive behaviors http://www.jsonline.com/alive/news/nov03/189263.asp?format=print
The researchers found that the same areas of the mouse brains that were activated when they were denied a chance to run also are known to fire up in drug-addicted mice that are denied their next fix.Vanderbilt is currently doing similar research on humans regarding sex/internet addiction. The results should be interesting, to say the least.(snip)
For the study, the mice were allowed to run all they wanted for six days. On the seventh day, they were not allowed to run.
The mice then were euthanized, and researchers checked levels of a protein that is an indicator of brain cell activation. Among the super runners, the levels of that protein were 42% to 63% higher, Rhodes said.
The higher levels were found in parts of the brain that are activated when drug-addicted mice are denied drugs such as cocaine, alcohol, nicotine and morphine.
The researchers said the higher levels of the protein show that the mice were highly motivated to run.