Read the paragraph and identify the way the paragraph illustrates. Doctors have recently started to study the causes of a medical disorder that they have appropriately named SAD, or seasonal affective disorder. People who suffer from SAD become very depressed during the winter months. Their depression appears to be the result of a decrease in the amount of sunlight they are exposed to. Doctors theorized that decreased sunlight affects the production of melatonin, a hormone manufactured in the brain, and serotonin, a chemical that helps transmit nerve impulses. Depression may result from the ensuing imbalance of these two substances in the body. Also, some doctors believe that a decrease in the amount of sunlight the body receives may cause a disturbance in the body's natural clock which could, in turn, result in symptoms such as lethargy, oversleeping, weight gain, anxiety and irritability — all signs of depression.