Autumn. Mornings are dark. Dusk comes dishearteningly early. You are feeling more tired, melancholy. The rapidly disappearing daylight seems almost to drag away part of your spirit with it. Should this dip in humor worry you? Not really--youll adjust. Unless you are prone to seasonal affective disorder. For the several million Americans who succumb, the darker half of the year brings a heavy veil of sadness. They become depressed, listless, chronically fatigued, and their mood does not rebound until March, when the daylight extends to early evening.