Brain scans reveal why sunlight powerfully influences mood

This winter was rough. Terrible things continued to happen, combined with the fact that it was dark. Sunday’s flip feels almost like a miracle. We sprang forward, allowing for an extra hour of daylight against the backdrop of an increasingly vaccinated world.

Winter will always be darker than summer — we can’t change Earth’s axial tilt. But we do have some control over the amount of daylight we’re exposed to. Daylight Saving Time (DST) was first formally implemented in 1916 by Germans, and came to the United States two years later.

It was standardized across the U.S. in 1966 via an act signed by Lyndon B. Johnson. The rationale was to uniformly establish when states pushed their clocks forward or back — rather than having states choose. However, in recent years, two states (Arizona and Hawaii) and several terriroties have voted to break free from DST. Meanwhile, other U.S. lawmakers are currently attempting to make it permanent.

The bipartisan-backed “Sunshine Protection Act” argues a permanent DST would improve mental health, public health, and public safety, citing the research suggesting these benefits would occur. In other countries, governments are trying to ban seasonal clock shifts altogether — no DST, just pick a time and stick with it. Ultimately, time is a construct and one singular fact remains: The Sun affects our brains, and in turn, our mood.

Why is this? — According to research published February in the Journal of Neuroscience, the answer links back to how daylight interacts with opioid receptors in the brain.

Lihua Sun is a postdoctoral researcher at the University of Turku in Finland and the study’s lead author. Opioid receptors are known to be linked with brain emotional functions, Sun tells Inverse. Sunlight is also known to influence emotions and social behavior. This is why Sun and his colleagues hypothesized sunlight and opioid receptor signaling were potentially coupled.

Opioid receptors are proteins on nerve cells, and the human body has three kinds of opioid receptors: mu, delta, and kappa. All of these receptors influence mood.

The study team examined how the length of daylight hours influenced those receptors in humans and rats. Ultimately, they found that the number of receptors they observed was dependent on the time of year the brain was imaged. This was especially true in the parts of the brain that control emotions and social behavior.

What exactly is going on, however, is a bit of a mystery.

“While we found a causal effect of daylength on brain opioid signaling, we have not addressed the exact mechanism,” Sun explains. “Previous knowledge of the effect of sunlight on the brain opioid system is very limited, and further research is required to understand this.”

Can daylight saving time affect your mood?

Sun hypothesizes these opioid receptor changes and their association with daylight hours could explain some of the science behind seasonal affective disorder (SAD).

In the future, he’d like to examine whether modulating the brain opioid system with drugs could mitigate SAD and “seasonality in brain cognitive disorders.”

Past research in mice suggests long daylength increases serotonin and norepinephrine — neurotransmitters that affect mood — and reduces anxiety and depression. It’s possible that a to-be-created treatment that interacts with opioid receptors could help brains in darker parts of the year.

Does all of this mean DST should be killed — or extended? It depends, Sun says. Because DST does increase the amount of time people are exposed to light, and his research suggests daylength can positively influence brain functions, there’s some argument to be made for extending daylight hours.

“However, for locations where daylength can be extremely long, like in Finland, my argument is different since daylength that’s too long may have an adverse effect by inducing physiological stress,” Sun says.

In Finland — DST begins March 28 there — summer days can mean 18 to 19 hours of light, which causes its own mental health problems.

Moderation, as usual, is best. Studies suggest you need 10 to 20 minutes of summer Sun to get all the vitamin D you need. Experience suggests you might want a bit more.