WHY DOES EXERCISE MAKE YOU FEEL GOOD?
Well, according to the latest research, exercising at least somewhat regularly--though it doesn't at all have to be intense, hours-daily exercise--keeps those neurons firing in your brain:
"Why does exercise make us happy and calm? Almost everyone agrees that it generally does, a conclusion supported by research. A survey by Norwegian researchers published this month, for instance, found that those who engaged in any exercise, even a small amount, reported improved mental health compared with Norwegians who, despite the tempting nearness of mountains and fjords, never got out and exercised. A separate study, presented last month at the annual meeting of the American College of Sports Medicine, showed that six weeks of bicycle riding or weight training eased symptoms in women who’d received a diagnosis of anxiety disorder. The weight training was especially effective at reducing feelings of irritability, perhaps (and this is my own interpretation) because the women felt capable now of pounding whomever or whatever was irritating them.
But just how, at a deep, cellular level, exercise affects anxiety and other moods has been difficult to pin down. The brain is physically inaccessible and dauntingly complex. But a recent animal study from researchers at the National Institute of Mental Health provides some intriguing new clues into how exercise intertwines with emotions, along with the soothing message that it may not require much physical activity to provide lasting emotional resilience.
For the experiment, researchers at the institute gathered two types of male mice. Some were strong and aggressive; the others were less so. The alpha mice got private cages. Male mice in the wild are territorial loners. So when then the punier mice were later slipped into the same cages as the aggressive rodents, separated only by a clear partition, the big mice acted like thugs. They employed every animal intimidation technique and, during daily, five-minute periods when the partition was removed, had to be restrained from harming the smaller mice, which, in the face of such treatment, became predictably twitchy and submissive.
After two weeks of cohabitation, many of these weaker mice were nervous wrecks. When the researchers tested them in a series of stressful situations away from the cages, the mice responded with, as the scientists call it, “anxiety-like behavior.” They froze or ran for dark corners. Everything upset them. “We don’t use words like ‘depressed’ to describe the animals’ condition,” said Michael L. Lehmann, a postdoctoral fellow at the institute and lead author of the study. But in effect, those mice had responded to the repeated stress by becoming depressed.
But that was not true for a subgroup of mice that had been allowed access to running wheels and nifty, explorable tubes in their cages for several weeks before they were housed with the aggressive mice. These mice, although wisely submissive when confronted by the bullies, rallied nicely when away from them. They didn’t freeze or cling to dark spaces in unfamiliar situations. They explored. They appeared to be, Dr. Lehmann said, “stress-resistant.”
“In people, we know that repeated applications of stress can lead to anxiety disorders and depression,” Dr. Lehmann said. “But one of the mysteries” of mental illness “is why some people respond pathologically to stress and some seem to be stress-resistant.”
To discern what was different, physiologically, about the stress-resistant mice, the scientists looked at brain cells using stains and other techniques. They determined that neurons in part of the rodents’ medial prefrontal cortex, an area of the brain involved in emotional processing in animals and people, had been firing often and rapidly in recent weeks, as had neurons in other, linked parts of the brain, including the amygdala, which is known to handle feelings of fear and anxiety.
The animals that had not run before moving in with the mean mice showed much less neuronal activity in these portions of the brain.
Dr. Lehmann said that he believed that the running was key to the exercised animals’ ability to bounce back from their unpleasant housing conditions.
Of course, as we all know, mice are not people. But the scientists believe that this particular experiment is a fair representation of human interpersonal relations, Dr. Lehmann said. Hierarchies, marked by bullying and resulting stress,  are found among people all the time. Think of your own most dysfunctional office job. (Interestingly, the same experiment cannot be conducted on female mice, who like being housed together, Dr. Lehmann said, so he and his colleagues are testing a female-centric version, in which “cage mates are swapped out continuously,” to the consternation and grief of the female mice left behind.)
Perhaps best of all, Dr. Lehmann does not believe that hours of daily exercise are needed or desirable to achieve emotional resilience. The mice in his lab ran only when and for as long as they wished, over the course of several weeks. Other animal experiments have intimated that too much exercise could contribute to anxiety, and Dr. Lehmann agrees that that outcome is possible. Moderate levels of exercise seem to provide the most stress-relieving benefits, he said. Dr. Lehmann does not have a car and walks everywhere, and although he lives in Washington, a cauldron of stress induction, he describes himself as a “pretty calm guy.”

I know in my case, I always do my most intense exercising in the morning, just after I wake up.  To me, it helps me with my metabolism, with (for example) working off last night's dinner.  It helps me wake up, and feel more ready to take on the day.  But maybe all those feelings are just a result of my neurons firing off and getting active...


"Act boldly and unseen forces will come to your aid."
-Dorothea Brande