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Seasonal affective disorder (SAD), aka seasonal depression, occurs in areas that experience lower levels of sunlight during certain times of the year. 

In the United States, it’s most prevalent in the northern regions of the country—i.e., Alaska, the Pacific Northwest, the Midwest, and the Northeast. The further north you live1 (i.e., the further you are from the equator), the more susceptible you are. 

According to a 2015 article from Depression Research and Treatment, SAD is four times more common in women than men and cases typically begin between the ages of 18 and 30. 

While seasonal depression is a clinical diagnosis, a subsyndromal type of SAD with milder symptoms called S-SAD, or “the winter blues,” is more common. For example, 15% of the Canadian population and 20% of the U.K. population experiences the winter blues, while only 2%-6% and 2% experience SAD in Canada and the U.K., respectively.

People with SAD have trouble regulating the synthesis of serotonin, aka the “feel good” hormone. Many also experience an overproduction of melatonin, the hormone that causes sleepiness when it gets dark. 

The combination of decreased serotonin and increased melatonin messes with the circadian rhythm, or the body’s “biological clock.” This effect is most intense during the months leading up to and following the winter solstice (i.e., November, December, January, and February), which is when days are the shortest and sunlight is least available.

What’s more, bundling up when we do head outside in the winter sun leaves little skin exposed for cutaneous vitamin D production. Vitamin D plays a role in the regulation of both serotonin and melatonin, making healthy D levels an important factor in whether or not SAD symptoms exist (or persist). 

Symptoms of SAD include:

• Sadness
• Tiredness
• Hopelessness
• Irritability
• Fatigue
• Having difficulty concentrating
• Inattentiveness
• Sleeping more than normal
• Lack of energy
• Social withdrawal

Seasonal depression affects certain individuals living in northern latitudes. Even more people in these locations are impacted by the winter blues each year. 

Whether you’re experiencing a form of clinical depression or simply feeling the effects of winter, talking to your health care provider about how to best support your mood during this time of year is an important step in caring for your mental well-being.