Seasonal affective disorder, also known as winter depression or SAD, was once regarded with skepticism by the scientific establishment. But there is now little question of its existence, as this New York Times article demonstrates.
It summarizes the history of research into the disease, and its association with melatonin and full-spectrum light. It also examines established treatments such as phototherapy and dawn simulation.
It also takes a look at the newest promising therapy for SAD -- negative air ionization. When this was used as a placebo in a recent study, it proved to have an actual, unexpected effect on mood! This may be in some way connected to the presence of negative ions in humid air, and their absence in dry air.