Earlier this week, autumn Daylight Savings Time begun, and although most of us gained an extra hour to sleep in this past weekend, the reduced exposure to natural light has been known to contribute to a mental disorder known as Seasonal Affective Disorder, or, as it is commonly referred to, “SAD.”

Many of us who live in the northern latitudes know that the strength and length of sunlight shrinks each year throughout fall and early winter. But it isn’t until the winter solstice (which, this year will be on Wednesday, Dec. 21), when the darkness has reached its maximum, will the daylight begin to gradually return, although most people don’t start to notice the prolonged light until late winter or early spring.

This lessened exposure to natural light may contribute to seasonal depression in some individuals.

Some of the symptoms of SAD include:

• Longer sleep

• Increased appetite

• Craving of sweet food

• Weight gain

• Difficulty concentrating

• Loss of interest in usual activites

“SAD is likely to have quite a complex etiology influenced by several variables,” writes Halszka Oginska and Katarzyna Oginska-Bruchal, two researchers from Poland who published a paper in the journal, Chronology International, which is titled, “Chronotype and Personality Factors of Predisposition to Seasonal Affective Disorder.” The researchers point out that variables like “genetic vulnerability, environment, socio-cultural context, and psychosocial factors” all help to influence seasonal depression.

Interestingly, SAD was not classified as a legitimate mental disorder until 1984, and when it was first included in the Diagnostic and Statistical Manual for Mental Disorders (DSM), it was included in as a sub-type under Major Depression Disorder (MDD), where it continues to remain. Also, other published research has suggested that SAD can even have a negative effect “on the offer price revision of an initial public offering (IPO) during bookkeeping.” In other words, put simply, seasonal affective disorder has been shown to influence the underpricing of stocks during an IPO.

So, the key question is: Why would the duration and strength of natural light impact a person’s mood?

Answer: Researchers think that SAD might be an evolutionary holdover from a more ancient time when the ancestors of humans hibernated.

“This extreme seasonality resembles an animal’s ability to hibernate in order to survive an adverse, cold season,” writes Oginska and Oginska-Bruchal. “It seems unwanted and useless in the contemporary world, a throwback of the evolutionary process.” In addition, a person’s personality type has also been liked to SAD. As it turns out, research has linked the personality trait of “openness [to new experiences]” to higher levels of SAD. “Openness appear[s] to be connected with the tendency of a lowered mood in the winter relative to the summer,” researchers reported.

Finally, if you begin to notice the development of symptoms mentioned above, and you think you might have SAD, it may be best to speak with a professional counselor or therapist.

The reduction of daylight during the winter months in the northern latitudes remains as inescapable fact of life. That said, however, much progress has been made understand and treat SAD since it was first identified more than 30 years ago.

References:

Oginska, H., and Oginsja-Bruchal, K. (2014). Chronotype and personality factors of predisposition to seasonal affective disorder. Chronobiology International, 31(4), 523 – 531.

Keef, S. P., O’Connor Keefe, M., Khlad, M. S. (2015). Seasonal Affective Disorder and IPO Underpricing: Updated Evidence. Journal of Neuroscience, Psychology, and Economics, 8(2), 78 – 99.