As the days grow shorter in fall and winter, many people experience low moods or seasonal affective disorder (SAD), a type of depression linked to reduced daylight.
But new research shows that the amount of actual sunlight—not just daylight hours—plays a much bigger role in mental health than previously thought.
A study by Dr. Shinsuke Tanaka from the University of Connecticut’s College of Agriculture, Health and Natural Resources has found that reduced sunlight exposure is strongly linked to higher suicide rates in the United States.
The research, published in the Journal of Health Economics, challenges previous findings that suggested suicides peak when daylight hours are longest.
Earlier studies reported that suicide rates often rise in the spring, when days are longer, leading many to believe that more sunlight might increase suicide risk. But Dr. Tanaka suspected that this pattern was misleading.
“The literature has shown a positive association between sunlight and suicide, and I thought that just doesn’t make sense,” he said.
“It’s got to be something related to the seasonality.”
Instead of simply tracking how many hours of daylight there were between sunrise and sunset, Tanaka took a more precise approach.
He used 25 years of NASA satellite data to measure the amount of solar radiation—the intensity of sunlight that actually reaches Earth’s surface—across U.S. counties.
This method accounted for cloudy, rainy, or hazy conditions that can greatly reduce the amount of sunlight people receive even when days are long.
Using this data, Tanaka found a clear pattern: the more sunlight people were exposed to, the lower the suicide rates. In fact, when sunlight exposure dropped by one standard deviation, suicide rates increased by about 6.8%. That impact is similar in magnitude to other major risk factors such as unemployment, access to firearms, or the presence of suicide prevention programs.
“This is the first empirical evidence to show a negative association between sunlight and suicide,” Tanaka said. “It highlights the mental health benefits of sunlight exposure.”
To support these findings, Tanaka also examined Google search data. He found that online searches for terms like “depression” and “suicide” rose when sunlight levels declined—further suggesting that low sunlight affects people’s mental well-being.
The study carries broader implications beyond personal health. Tanaka warns that future climate technologies, such as solar geoengineering—which would reduce sunlight reaching Earth to combat global warming—might have unintended consequences for mental health. “By reducing sunlight, we show that there is this mental health problem,” he said. “Our findings highlight the potential cost of adopting solar geoengineering.”
As modern lifestyles keep people indoors for much of the day, Tanaka’s research serves as a reminder of nature’s simple prescription for mental health: get outside and soak up some sunlight whenever you can.
If you care about mental health, please read studies about cannabis use disorder linked to increased risk of mental diseases and some mental health drugs can cause rapid weight gain.
For more health information, please read studies that one sleepless night can reverse depression for days and scientists find better treatment for older adults with depression.
Source: University of Connecticut.
