It turns out there may be more to that uncomfortable nagging feeling that avid exercises experience if they miss a workout. While some people seem to look for any excuse to dodge the gym or a run, there is another group just as anxious to get their workout in no matter what. Of course this is usually the case with professional athletes and other high caliber competitors as it is usually vital for them to stick to a rigid training schedule; however there are plenty of 'regular' people who seem almost addicted to their daily sweat regime.

New research from Tufts University backs up what these people can only explain as simply something they 'have' to do to feel 'right'. It's not uncommon to find runners who are sent into a sort of gloom or despair at having missed a run, and even running 'streakers' who boast of not missing a single day of running for years on end. That fabled 'runner's high', while regarded as a sort of elusive holy grail in the running community when the act is so effortless that you feel you could go on forever, may actually have a physical cause.

Endorphins, which are released by the pituitary gland and the hypothalamus during times of intense exercise in reaction to the body's depleted glycogen levels, are interpreted by the brain similarly to those found with opiate use. These endorphins produce a positive feeling to the exerciser and thus create the feeling of ease and unending energy. It is in this same vein, that many recovery drug addicts substitute exercise during times of intense cravings during their recovery.

When rats, which are often found to parallel reactions in humans during tests, were allowed to run freely on a wheel for a few weeks and then abruptly forced to stop, they experienced signs of withdrawal symptoms. The team conjectured that this same response is what eager exercisers of the human variety claim to be that itch to get in their workout no matter what. "Exercise, like drugs of abuse, leads to the release of neurotransmitters such as endorphins and dopamine, which are involved with a sense of reward," Robin Kanaraek, the professor of psychology leading the project stated.

Of course, if one has to pick an addiction, exercise, when kept below an dangerously extreme level sometimes found in eating and exercise disorders, is a relatively benign one to harbor. Keep that in mind the next time you kid your friend when they skip on other plans in favor of a date with the gym, and try not to take it personally.