A 2000 study published in the New England Journal of Medicine suggests that   we probably only gain about a pound or two during the winter season. The trouble   is that this extra weight accumulates through the years and can be a major contributor   to obesity later in life. Granted, it's only a pound, but winter doesn't have   to be a bulking phase:

The Dark Side
  In nature, winter was always a time of low food supply when all animals would   rely on the fat pad that they had built up during the summer months. Those with   low fat stores would be at higher risk of starvation and death, so our bodies   evolved an acute insulin response to efficiently store carbohydrates. Long summer   days meant continuously elevated insulin to store sugars from fruits, starches   from tubers and caramel from your mocha latte. To mammals, including humans,   continuously elevated insulin meant only one thing: winter was coming

It's Always Sunny in Philadelphia
  Through the miracle of technology, we've eliminated our traditional famine period.   Light bulbs and computer screens provide the artificial daylight, and year-round   access to carbs provides the food security. We have confused our bodies into   thinking that summer is still here, and, frankly, you can't blame us. Summer   is a time of feasting, frolicking and fornicating. Who wouldn't want that to   last all year? But Mother Nature isn't easily fooled. In return for banishing   darkness, we suffer from weight gain, seasonal affective disorder, depression,   lower immunity, higher stress, and fatigue, all of which falls under the scientifically   accurate term "winter blues."

Step 1: Feed Your Hunger For Sleep
  Light and dark cycles control insulin through carb cravings but also, more directly,   through your stress mechanisms. When the lights are on, your cortisol levels   stay up because you need the ability to fight, run and get stuff done. Cortisol   mobilizes blood sugar, which means that insulin also stays up to disperse that   sugar to your muscles. So staying up to watch Letterman keeps your insulin up   longer than nature intended, and that means one thing: You get fat just by smelling   a cookie. As a result, we crave carbohydrates only when we're tired, not when   we need food. More and more research shows that chronic sleep deprivation leads   to weight gain. So turn off the TV by 9 p.m., get to bed by 10 p.m. and try   to get as much sleep as you can without getting fired or divorced.

Step 2: Eat Fewer Carbs
  If you can accomplish Step 1, you'll lose weight naturally and be able to resist   carbs more easily. Sugar is an addictive white powder because it raises levels   of serotonin in our brains. (Serotonin is a feel-good hormone that is involved   in eating and energy balance and emotional functions like mood.) Researchers   recently discovered that serotonin transporters are significantly higher in   the winter, which means that serotonin is cleared away faster. This is the biological   explanation for the winter blues. The reason this happens is because in nature   winter carbs are somewhat hard to come by, so our bodies have evolved to survive   on a seasonal low-carb diet. The only winter carbs cavemen had access to were   root vegetables and bark. So choose carrots, potatoes, onions, leeks, pumpkin,   and turnips instead of pasta, breads and grain cereals. And drop the sugar --   the stuff wasn't even considered a food until the modern era.

Step 3: Eat more protein and fat
  After the late-harvest green vegetables were eaten, our bodies would prepare   for the coming winter months by progressively relying on free fatty acids instead   of straight glucose. Fats can replace carbohydrate energy for virtually every   metabolic process. That's how bears survive hibernation. Fats and proteins don't   raise insulin like carbs do, so you'll naturally lose weight. Research also   suggests that doing so would give your heart a rest from all those nasty free   radicals produced during carb metabolism, which is why the Inuit never had heart   disease or diabetes before they started eating modern foods. Besides, meat is   a great source of zinc, an essential nutrient for proper immune function.

Step 4: Go outside For Your Vitamin
  Levels of the sunshine vitamin are notoriously low when people don't get enough   ultraviolet-B (UVB) light on their skin. A recent study conducted at the University   of Minnesota found that overweight folks have better success at losing weight   when their vitamin D levels are increased. Vitamin D is essential for absorbing   calcium from our foods, and a deficiency in one means a deficiency in the other.   Low calcium causes a 500-fold increase in the production of an enzyme that converts   energy into fat, causing you to pack on the pounds.

Step 5: Cut the comfort Coffees
  At least seven scientific studies provide strong evidence that calorie-high   beverages do not properly activate the satiety mechanisms in the body and brain   and do not satisfy the appetite as well as solid food. This makes sense from   an evolutionary perspective. Nature did not prepare us to process liquid calories   since we drank water for most of human history. High-sugar drinks didn't exist   until 150 years ago, and they weren't consumed in significant amounts until   the past 50 years.