Think of it as a form of hibernation. We, too, have retained our intrinsic seasonal responses. The treatment for SAD comes directly from research in mammalian physiology. While current antidepressants are useful for some SAD sufferers, light therapy is even more effective. Full-spectrum light at 10,000 lux for thirty minutes a day has been shown to be even more effective than medication.
Timing also makes a difference, with morning better than evening exposure.
Actually, there is a more natural, and for my prescription pad, preferable treatment for Seasonal Affective Disorder.