A recurrent winter depression is brought on in some people by the deprivation of light which exists in northern latitudes around the time of the winter solstice. Shorter, darker days seem to dampen production of the mood-boosting brain chemical serotonin in SAD sufferers and elevate levels of melatonin, the sleep-inducing hormone produced at higher rates in the dark. Called seasonal affective disorder (SAD), it is triggered by lack of bright light penetrating the retina to the pineal gland in the brain. Darkness produces the sleep-inducing hormone melatonin, which makes people lethargic, oversleep, crave carbohydrates and feel depressed. Clinical symptom are: sluggishness, fatigue, irritability, depression, feelings of loneliness, anxiety and increased appetite/weight gain. Exposure to strong, continuous artificial light sources can immediately alleviate the symptoms, but the disorder has no known cure.
A rarer form of seasonal depression occurs during the spring and summer months with its own set of symptoms, including insomnia and weight loss.
SAD affects about 4 to 6 percent of the population, with another 10 to 20 percent experiencing a milder form. Women are more likely to experience SAD (four out of five sufferers are female), as are individuals prone to non-seasonal depression.
In the USA it has been estimated that the incidence of SAD rises with increasing latitude: from 1.4% in Florida to 9.7% in New Hampshire. Studies indicate that women sufferers outnumber men by 3 to 1. There is evidence to indicate that women sufferers have a high rate of premenstrual tension syndrome.