Lack of physical exercise is the primary factor for the prevalence of depression. With passage of time, man has evolved from a physically challenged creature to one who is mentally burdened. The absence of physical challenge and overwhelming abundance of mental work is forcing more and more people to become victim to depression.
When a person is engaged in work requiring long hours of concentration, the brain produces large quantities of neurotransmitters. These neurotransmitters enable the person to perform work effectively. At the same time, the large release of these chemicals also increases receptor density. Once a person completes his or her work the quantity of chemicals returns to normal levels. The increased density of neurotransmitter receptors, however, does not decrease. Here, receptors now need increased levels of chemicals to produce effects as those produced prior to the increase in density.
This can be best understood by means of the following example. If you keep your hand submerged in cold water, the water will feel less cold as the length of time of contact increases. This is a short term adaptation of the body to change. Now if you return after a few days to water of the same coldness and submerge your hand, you will again feel cold. Repeating this process for several months will force your body into long term adaptation. You will no longer find the water cold after a few months even on initial contact. Instead, you will need to lower the temperature of the water to feel cold.
Similarly, increase in neurotransmitter release in the brain over an extended period of time increases density of receptors. Our body has receptors which respond to specific chemicals. The response translates into different effects depending on the chemical and receptor. For example, when the body produces a chemical known as epinephrine, the chemical attaches to epinephrine-receptors stimulating the receptor to cause an increase in heart rate.
The chemical mainly believed to be responsible for mood regulation is serotonin. An increase in serotonin during mentally challenging work leads to an increase in density of serotonin receptors. Now, serotonin receptors require increased levels of the neurotransmitter to produce their effect. The body, however, does not produce increased levels of neurotransmitter once work is over. The result is depression.
In order to circumvent this situation, people need to incorporate exercise into their daily routine. Exercise results in a massive release of chemicals in the body. This helps to stimulate receptors which, due to increased density, display diminished responsiveness to the normal levels produced by the body. This release continues over an extended period of time helping to alleviate symptoms of depression.