How can exercise change the we feel? Deep inside our skull, we have a "brain within the brain" that is responsible for emotions. This so-called limbic region also balances heart rate, blood pressure, digestion, and all the hormones of the body. Because of such neural multitasking, when our body changes, our emotions change, too. During exercise, for example, our body releases endorphins—tiny molecules that resemble opium and its derivatives (like morphine and heroin). The emotional brain contains many receptors for endorphins, and that's why it is so sensitive to opiates. (The drugs hijack the pleasure-registering receptors, immediately radiating a sensation of well-being and satisfaction.) When opiates are used often, however, they can become habit forming. The flooded brain reacts by reducing the number of receptors in order to avoid overstimulation. So the dose must be systematically increased in order to produce the same response. Moreover, because the receptors become less and less sensitive, regular pleasures lose their potency—including sex, which for drug addicts often offers little or no enjoyment.
The secretion of endorphins brought on by physical exercise does exactly the opposite. The more the natural mechanism of pleasure is gently stimulated by exercise, the more sensitive it becomes. In addition to relishing sex and life's other big rewards, people who exercise regularly actually get more pleasure out of the little things: their friendships, cats, meals, hobbies, even the smiles of passersby in the street. Essentially, it becomes easier for them to be satisfied. Such heightened enjoyment is the antithesis of depression.
Stimulating the emotional brain through exercise also promotes the proliferation of the immune system's natural killer cells, making them more aggressive against infections and cancer. In addition, people who exercise show a greater balance in heart rate control than people who do not. All the symptoms of anxiety start with an overactive sympathetic nervous system, which releases adrenaline and causes an increase in heartbeat, blood pressure, sweating, and trembling. With regular exercise, the parasympathetic nervous system—the physiological "brake" that slows all these reactions (and works in opposition to the sympathetic system)—becomes healthier and stronger. Eventually, it simply blocks the symptoms of anxiety.
Even soft forms of exercise such as yoga and tai chi have been found to help elevate mood. The 5,000-year-old yoga tradition recommends specific postures (asanas) to fight off depression. In one study at the University of California, Los Angeles, just two hours a week of yoga practice enhanced mood and reduced anxiety in less than a month. In a study of tai chi at the Oregon Research Institute, a nonprofit primarily funded by the National Institutes of Health, people ages 65 and older who attended two one-hour classes a week were in a much brighter mood compared with a control group after six months. And typically those who do the least amount of physical activity are precisely the people whose moods are the most improved through exercise. This is one of the curious things about exercise: The less fit we are (the richer our meals, the more time we spend in front of TV or behind the wheel of a car), the more noticeable its benefits, even in small doses. What's essential is to find a form of activity that feels right to us and that we soon start looking forward to.
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