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As the days get shorter and the chilly weather rolls in, we all want to curl up in a blanket and hibernate until spring rolls around. But making time to get outside in the sun, even when it’s cold out, could have bigger mood benefits than you might realize.

While the link between sunshine and mental health is nothing new, the most comprehensive study to date has shown that the association may be even stronger than previously realized.

New research from Brigham Young University, published this month in the Journal of Affective Disorders, finds that sunlight exposure is by far the greatest weather-related factor determining mental health outcomes. In other words: more sunshine, more happiness.

For the study, a psychologist, a physicist and a statistician from BYU teamed up to compare daily environmental data from the university’s Physics and Astronomy Weather Station with emotional health data archived by day for 16,452 adult therapy patients who were being treated at the BYU Counseling and Psychological Services Center.

If you’re getting enough sun, your emotions should remain relatively stable, the researchers found. But as the amount of sunlight in the day is reduced, levels of emotional distress can shoot up. Other weather variables including temperature, pollution and rain were not found to have an impact on mental health.

“We were surprised that many of the weather and pollution variables we included in the study were not significantly correlated with clients’ scores on the distress measure once we had accounted for suntime,” Dr. Mark Beecher, a professor of psychology at the university and the study’s lead author, told The Huffington Post. “People tend to associate rainy days, pollution, and other meteorological phenomena with sadness or depression, but we did not find that.”

Exposure to sunlight is a significant factor in seasonal affective disorder. Research has shown that the brain produces more of the feel-good neurotransmitter serotonin on sunny days than it does on darker days. What’s more, lack of sunlight is linked with lower vitamin D levels, which in turn has been correlated with depression and low energy. However, the findings aren’t only relevant to those suffering from SAD, as the research was focused on a non-clinical population.