Walking on Sunshine: The Light of Day Improves Mental Health

Brains evolved on a planet that spins in the presence of a bright sun. During this evolution, brains utilized the distinct light-to-dark rhythm by aligning specific biological and behavioral functions with waking up each day. These circadian rhythms are fundamental to general mental and physical health. If we disconnect from this rhythm our mood declines as our sleep quality becomes impaired. Life in the twenty-first century has dramatically altered our ancient link to this rhythm. Most of our waking hours are spent inside with artificial lighting conditions and reduced sunlight exposure. Complicating matters further, too many of us experience too much light at night. Subsequently, too many of us suffer from chronic low mood, fatigue, and poor sleep.

A recent study investigated whether exposure to sunshine could reduce depressive symptoms. The authors examined the associations between self-reported time spent in outdoor light during the day and mood and sleep quality. The participants were 502,000 adults, aged 37–73 years; 54 percent were women. The median time spent in the daylight was 2.5 hours each day. The results showed that each additional hour spent outdoors during the day was associated with significantly lower odds of developing a lifetime major depressive disorder, reduced antidepressant usage, less frequent anhedonia (the inability to experience pleasure), less frequent low mood, greater happiness, and lower neuroticism. These positives were independent of any demographic, lifestyle, or employment variables.

The effects of sunshine exposure held longitudinally, meaning that exposure to sunshine at the first time point predicted better mood outcomes at the second time point. Daytime light exposure may improve mood by correcting abnormal circadian timing. Beyond its effects on our internal clock, the authors speculated that daytime sunshine may influence areas of the brain involved in emotional regulation.

How does sunshine influence the brain? Unlike some amphibians and reptiles, sunshine is not able to penetrate directly into our brain through a hole in the top of the skull. Our hair, skull, and cortex get in the way. Humans, as well as most other vertebrates, have a small number of specialized cells in our retina that are sensitive to sunshine. These specialized light-sensitive cells communicate with multiple brain regions that are associated with the control of circadian rhythms as well as the control of mood. (If you would like to learn more about how the brain “sees,” please see the reference below.)

Spending more time outside in the sunshine was also associated with greater ease of awakening earlier in the day. Scientists have long suspected that exposure to light in the morning shifts our circadian rhythms earlier. In contrast, exposure to light at night shifts our circadian rhythms later.

In general, greater daytime light exposure predicts better sleep quality, fewer insomnia symptoms, better mood, and less fatigue.