The way your body reacts to the seasons is controlled by a very old and primitive part of the brain that is about the size of a walnut. It’s called the hypothalamus.

Janis Anderson, director of the Seasonal Affective Disorders Clinical Services at Brigham and Women’s Hospital, likens it to a cluttered central clearing house, not unlike a janitor's closet.

Anderson says that in the dark corner of the closet in your brain, the hypothalamus is hard at work, keeping the “physical plant” alive—regulating body temperature, metabolism, hunger and thirst—and keeping time on your biological clock. And it knows not just what time of day it is, it knows the time of year by keeping track of light during the day.

The hypothalamus bridges the largely rational life humans live today with the more instinctual existence we led thousands of years ago when we were closer to our less evolved origins. 

“Now here in Boston,” Anderson points out, “we’re getting to the shortest days of the year…  we will have approximately nine hours of light, and 15 hours of darkness.” 

So the hypothalamus, always making adjustments, makes some bigger adjustments: changes to your neurochemistry that can affect energy, mood, and appetite. 

“The changes in our sleep and eating patterns are quite reminiscent of other organisms that are preparing for the wintertime,” says Robert Levitan, who heads depression studies at the Center for Addiction and Mental Health at the University of Toronto. 

Levitan continues: “I would say it’s not exactly hibernation but shares a lot of what it accomplishes, which is energy conservation, and that protective aspect to it.” 

But if you’re a mammal who’s lost the fur, moved indoors and invented artificial heat, light, and agriculture—um, us— these adjustments may not be so helpful. 

Levitan goes on to catalogue that classic symptoms of the winter blues: “Loss of energy, loss of interest in social interactions combined with energy conserving symptoms, both overeating, craving for carbohydrates fats, often significant weight gain, tendency to sleep a lot more than usual.” 

Since we all go through these brain changes in winter, people can experience anything on the continuum from a bit of winter lethargy, to full-on Seasonal Affective disorder, which can be debilitating. 

So what can you do to fight against your own brain? 

Well first, if seasonal depression is disrupting your life, see a doctor.   

Light therapy—with the right kind of illumination—has been shown to be effective, as have other treatments often used for clinical depression. 

There are also some habits that can help. 

For one: Take it easy on those carbohydrates you’re craving.  Won’t help your energy. 

Next, Janis Anderson says: Get out of Bed. 

He cautions against sleeping in. It’s a myth, says Anderson, that you should hibernate like a bear.

Even though human biological processes are akin to seasonal hibernation in other mammals, fight that urge.

“Get out, get active and stay engaged,” advises Anderson. 

Anderson points to studies that show certain native populations tend to suffer less from seasonal depression than other populations at the same latitude.

There’s probably a genetic component, but Anderson says their happiness might be a result of their traditional lifestyle, with lots of outdoor activities, and culture. 

So if abstaining from carby holiday delicacies and sleeping in strike you as going against the holiday spirit, take comfort in the fact that you can feel good about partying with your friends.  It’s good for your health.