Your bathroom scale isn't lying: You really are gaining winter weight. Consider it an unmistakable reminder that long before we were regularly bombarded by ads featuring the immaculate abs of celebrities and multi-day cleanses that taste like grass clippings, our ancestors needed those extra couple of pounds to protect them against the season's inclement weather. From an evolutionary standpoint, it's why that extra helping of pasta, that greasy slice of pizza, or even that stale, sprinkled donut all appear extra tempting when the temperature drops a few degrees. 

Indeed, Dr. Norman Rosenthal explains at Psychology Today that winter's waistline-sabotaging ways may have something to do with a brain chemical that prods us into craving carbohydrates:

One likely explanation involves the brain chemical serotonin, which, during the winter months, falls to its lowest levels in parts of the brain that regulate mood and appetite. Research shows that on sunny days the brain produces more serotonin; on dark days less. Another way to boost brain serotonin (besides bright light) is by eating sugary or starchy foods, which causes insulin to be secreted. This, in turn, pushes tryptophan (a crucial building block for serotonin) from the bloodstream into the brain. Unfortunately, the secretion of insulin drives down our blood sugar, making us hungry for more sugary and starchy foods. [Psychology Today]

The desire for processed starches and carbs, as we've come to learn, is akin to an insidious little shoulder devil that makes it difficult to keep our muffin tops in check. It's why high-protein, low-carb diets — like Atkins or Paleo — have become as popular as they are. It's also why for all their legitimate concerns, both diets have their merits.

Winter's dearth of sunlight doesn't just make you hungry; it makes you lazy too. When it's dark, your body produces more of a hormone called melatonin, which is critical for moderating your body's internal clock. It's why we often find our feet dragging on gloomy winter days, and it's why you increasingly feel okay with getting to bed around 9 from December to March. Some experts think melatonin also plays a role in increasing our appetites, which, when paired with winter's cozy allure of a sedentary lifestyle, can cause the pounds to pack on.

Then there's Vitamin D. The body really only manufactures it when there's plenty of sun out. That's why the so-called "sunshine vitamin" is abundant in the summer. But Vitamin D has also been linked to appetite control and burning fat. During the winter months, you're not only outside far less, but you're also bundled up from head to toe. That means way less Vitamin D.

And let's face it: All that snow and ice outside doesn't make it easy to stay motivated. Running outside is hard when the sidewalk is icy. The gym is packed with people clinging to New Year's resolutions. And all those extra layers hide our body shapes whenever we look into the mirror for guidance.

So, no, winter's temperature drop isn't kind to your waistline. At all. A few extra pounds aren't the worst thing in the world, but it's worth keeping in the mind that while the body's natural mechanisms helped our ancestors survive the brutal cold, sitting idly in a temperature controlled office snacking all day probably isn't what Mother Nature intended, either.