Winter presented a unique problem for our early human ancestors. Unlike mammals that hibernate until spring, we had to plan ahead, secure shelter and find ways to preserve food stocks and other resources to get through the harshest months of the year.
Why, then, would we have evolved something like seasonal affective disorder? It must have served some adaptive purpose. Evolution would have favored ancestors that had a tendency to sleep more, eat less, and in general, use up less resources than others during these months. At a time when food is most scarce, this might have made the difference between living to see spring or ending up a pile of bones in a Werner Herzog film.
Numerous studies have shown that depression encourages a kind of ruminative deep thinking that is highly analytical in nature and effective in problem solving—a useful adaptation for the resource-scarce winter months. Other symptoms, like loss of appetite and lack of interest in sex, would have served the purpose of conserving energy and limiting unnecessary social conflict that might have otherwise risked the survival of the group.
People in depressed mood states are also better at solving social dilemmas. Laboratory experiments indicate that depressed people better analyze the costs and benefits of the different options in a given situation. Living in tightly-knit cave communities in the winter, successful navigation of social dynamics would have been an essential survival skill. To make a grave social mistake and be banished from the group during these months would have meant certain death.
That said, most of us aren't living in caves anymore. Like any strain of depression, seasonal affective disorder can negatively impact our relationships with our friends, family, and impact our careers. Depressive episodes that start in the winter can snowball into several months, leaving people socially isolated, lethargic, and unable to derive pleasure from basic activities like eating and sex. At its most dire, it can even have life-threatening implications.
If you think you're susceptible to seasonal depression, here are some easy ways to keep the winter blues at bay (courtesy of PubMed Health):
1. Light therapy
Light therapy using a special lamp with a very bright fluorescent light (10,000 lux) that mimics light from the sun may be helpful. It's most commonly used in the mornings to mimic sunrise, and just 30 minutes a day can be enough to stave off depressive symptoms.
2. Get outside whenever possible
Taking long walks during the daylight hours has been shown to help patients adjust to vitamin D deficiency during winter months.
3. Get plenty of exercise
As with every kind of depression, exercise releases natural endorphins that can boost mood and energy levels.
4. Keep active socially
It may involve some effort, but keeping a reasonably active social life during the winter months can do wonders for your mood.
5. Antidepressant / Talk therapy
As with other types of depression, medication and therapy can help ameliorate symptoms. Always consult a doctor before taking any medication of any kind.