If you find yourself becoming depressed, increasingly irritable and suffering from more bouts of insomnia as winter approaches, you may have seasonal affective disorder (SAD). Your risk of having this condition increases the further you get from the equator.
The prevalence of SAD is estimated to be only 1.4 percent in Florida, but jumps to 9.7 percent in New Hampshire. Although it is not listed as a mental disorder in itself, it is categorized as a specific type of depression.
Dr. Norman Rosenthal was the first researcher to study and name this phenomenon, motivated by his desire to understand what caused his depression in the long dark days of the northern winters. Rosenthal and colleagues conducted a placebo-controlled study of SAD that used light therapy, which was found to be effective in alleviating some of the symptoms of SAD.
Although the exact cause of SAD is not fully understood, scientists believe that the hypothalamus is negatively affected by the relative lack of sunlight in more northern latitudes (or more southern latitudes for people in the southern hemisphere). The hypothalamus regulates our circadian rhythm and produces the hormones that influence sleep, mood and appetite.
Some of the most common warning signs of seasonal affective disorder include can be found here