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Seasonality is reported by approximately 10 to 20 percent of people with depression and 15 to 22 percent of those with bipolar disorder. “People often don’t realise that there is a continuum between the winter blues—which is a milder form of feeling down, [sleepier and less energetic]—and when this is combined with a major depression,” says Anna Wirz-Justice, an emeritus professor of psychiatric neurobiology at the Centre for Chronobiology in Basel, Switzerland. Even healthy people who have no seasonal problems seem to experience this low-amplitude change over the year, with worse mood and energy during autumn and winter and an improvement in spring and summer, she says. - arstechnica.com