Less Light, More Blues: Unraveling the Science Behind Seasonal Affective Disorder
Less sensitive light receptors in the eyes are linked to seasonal affective disorder.
Researchers recently confirmed that one of the differences in people with seasonal affective disorder (SAD) is that their eyes are not as sensitive to light as people without SAD. SAD affects about 5% of the population, particularly during the winter months. We summarized the frequently asked questions about SAD in a previous blog post, where we said that while we know less winter light triggers SAD, no one actually knows what causes SAD.
The light-detection cells in the eyes of people with SAD are not as responsive in the winter.
Now, scientists have found a biological mechanism behind the winter blues. SAD is linked to a light-sensitive cell in the back of the eye called the intrinsically-photosensitive retinal ganglion cell (ipRGC). The ipRGC is designed to simply detect light, unlike cones and rods which help us see colors, shapes, and movement. The more light the cell detects, the more the pupil constricts. By measuring pupil constriction in 65 study participants, Dr. Kathryn A. Roecklein and her colleagues found that people with SAD aren't as responsive to light in the winter months, compared with people who don't have SAD.
More light helps people with SAD.
Bright light has been a cornerstone of treatment for SAD, and now we finally have a reason for why it works. People whose ipRGC are less sensitive to light are more prone to developing SAD. Bright light exposure for SAD helps patients overcome their less responsive light-detection ipRGC.
SAD often starts when daylight savings in the fall.
The ipRGC's impact on SAD also explains why so many cases of SAD start soon after daylight saving ends in the fall, especially for night owls. People on a slightly later schedule have less light exposure throughout the day as they lose the hour of daylight early in the morning. It gets dark by the time work ends so it's difficult to have much sun exposure at all. Without strong light stimuli, people are more prone to developing SAD.
U.S. States with the highest and lowest rates of SAD correlate with latitude.
The amount of light exposure is central to SAD. The higher the latitude, the higher the rates of SAD. In the winter, the Earth tilts away from the sun. The further North you go, the further away the Earth is from the sun. The Northernmost U.S. town of Utqiagvik, Alaska doesn't receive any sun for 66 days. In contrast, during the winter solstice in Hawaii, there are still 11 hours of daylight. People in Alaska, Oregon, Washington, and Idaho have the highest rates of SAD in the US. Hawaii, North Carolina, Florida, and Arkansas have the lowest rates of SAD. Daylight reduces the rate of winter blues.
SleepPhones® headphones sleep aid sales rates correlate with latitude.
In a recent analysis comparing SleepPhones® headphones sales to latitude, we also showed a strong correlation between higher latitudes and higher sales of our sleep aid. For example, our data shows that per capita, Alaska buys 3 times more SleepPhones® headphones than Florida. For every 10 degrees increase in latitude North, our sleep aid sales increase by 12.5%. Read Alaska vs. Hawaii: The Sleep Struggle Is Real (and Daylight Saving Doesn't Help) for more information correlating sales and latitude. To make it even more clear, we sorted the states by latitude. The warmer the color, the higher the sales. For example, Alaska is in red because it has among the highest sales per capita. The bluest states have the least sales per capita. When sorted by latitude, the color gradient makes the association obvious.
Should we all move to Hawaii?
There certainly seems to be a benefit to mental health when comparing Hawaii's sunshine to Alaska's winter darkness. Fear not! SleepPhones® headphones help to combat the poor sleep quality associated with less sunlight. Sleeping better improves mood and perhaps SAD. Many people have already discovered the benefit of SleepPhones® headphones and preferentially buy more in the winter, especially those in the states most affected by the tilt of the Earth away from the sun. Do you know someone living in the Northern latitudes who could use more sleep?
References:
https://www.sciencedirect.com/science/article/abs/pii/S016503272100416X
https://academic.oup.com/sleep/article-abstract/47/6/zsae066/7623292
https://pmc.ncbi.nlm.nih.gov/articles/PMC4673349/
https://www.forbes.com/health/mind/winter-blues-states/
Find more about seasonal affective disorder (SAD) here.