Winter blues

A health column in the local paper described Seasonal Affective Disorder (depression that comes with the long nights of winter) in this manner:

It's a variant of bipolar disorder, except that they don't get manic.

That's like describing vanilla ice cream as like neapolitan ice cream, except without chocolate and strawberry.

To a layman like me, SAD is a variant of depression. Bipolar disorder is manic-depression, and without the two extremes, it is not a valid comparison.