I guess its only one D, SAD seasonal affective disorder. Its when you live someplace where the sun doesnt shine for long periods of time and it causes depression.
Its usually a winter time ailment because the days are shorter and in some areas of the country the sun doesnt shine for days at a time. I think its fairly common in the pacific northwest where it rains a lot and in alaska where they have only a few hours of daylight during the winter months.
I would love Alaska during the midnight sun time. I am one of those that sleep only a few hours a night so no problem there.