These symptoms are signs that you may be experiencing seasonal affective disorder — or SAD, a type of depression that usually occurs during the winter. Indeed, as many as one in five Americans have SAD, and 75 percent are women, according to the American Psychiatric Association.
What causes SAD? Sunlight enters the brain through the eyes, stimulating the production of a neurotransmitter, serotonin, that supports nerve cell functioning, including mood, according to the Mayo Clinic. Less light results in lower serotonin levels. Darkness prompts the production of melatonin, which promotes sleep. It’s the combination of less serotonin and increased amounts of melatonin that results in SAD.