In a few days, the winter solstice will plunge us into the longest and darkest night of the year. Is it any surprise that we humans respond with a holiday season of relentless cheer and partying?

It doesn’t work for everyone, though. As daylight wanes, millions begin to feel depressed, sluggish and socially withdrawn. They also tend to sleep more, eat more and have less sex. By spring or summer the symptoms abate, only to return the next autumn.

Once regarded skeptically by the experts, seasonal affective disorder, SAD for short, is now well established. Epidemiological studies estimate that its prevalence in the adult population ranges from 1.4 percent (Florida) to 9.7 percent (New Hampshire).

Researchers have noted a similarity between SAD symptoms and seasonal changes in other mammals, particularly those that sensibly pass the dark winter hibernating in a warm hole. Animals have brain circuits that sense day length and control the timing of seasonal behavior. Do humans do the same?