The Reason You're Sad This Winter (and It's Not Seasonal Affective Disorder)

To be human is to be sad in the winter. Those post-holiday, low-sunlight, cold-weather vibes hit in early January, and by default, we often attribute our depleted mood to seasonal affective disorder—a buzzy, though very real, condition caused by hormonal changes and a lowered serotonin production in the wintertime. However, there's actually a markedly less talked-about but even more real culprit for your wintertime sadness: vitamin D deficiency.

We've all heard of vitamin D, but here's a refresher on what it is and what it does. It's is a fat-soluble vitamin responsible for calcium homeostasis and bone health, explains Tsippora Shainhouse, MD, a board-certified dermatologist and pediatrician in Los Angeles. Healthy levels of vitamin D are shown to slow skin aging, promote healthy immune function and bone growth, and improve mood. According to Rachel Fine, a registered dietitian and owner of NYC nutrition counseling firm To the Pointe Nutrition, signs of vitamin D deficiency may include frequent colds, lower back pain, chronic aches, and fatigue and depression.