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. Seasonal affective disorder (SAD) 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. Women sufferers outnumber men four to one. Exposure to strong, continuous artificial light sources can immediately alleviate the symptoms, but the disorder has no known cure.
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.
In 1998, it was claimed that very small doses of melatonin could relieve SAD.