Sleep regularity, such as having a consistent sleep schedule, may help explain the mood-boosting effects of sunlight or bright light exposure.

Summary: A study conducted by Brigham and Women’s Hospital found that spending more time in bright light, akin to daylight exposure, is associated with improved sleep regularity, which in turn correlates with lower depression symptoms and reduced odds of mild or severe depression. Using data from over 6,600 participants, the researchers also observed a link between bright light exposure, higher vitamin D levels, and better sleep regularity, though vitamin D was not directly associated with depression symptoms. While the findings highlight the potential benefits of bright light and sleep consistency for mood, the study’s cross-sectional design limits conclusions about causality.

Key Takeaways:

Bright Light Improves Mood Through Sleep Regularity: Participants exposed to more bright light had more regular sleep patterns, which were linked to lower depression symptoms.
Vitamin D and Sleep Play Separate Roles: Bright light exposure increased vitamin D levels and improved sleep regularity, but only sleep regularity showed an association with reduced depression symptoms.
Causality Needs Further Study: The cross-sectional nature of the data means it’s unclear whether bright light exposure directly reduces depression or if other factors, like depression symptoms, influence time spent in light. Future longitudinal studies are needed.

Why might more time in the sun boost a person’s mood? A new study led by investigators at Brigham and Women’s Hospital, a founding member of the Mass General Brigham healthcare system, suggests that sleep may hold the key.

The study, which included more than 6,600 participants, found that participants who spent more time in bright light had more regular sleep, and more regular sleep was associated with lower depression symptoms and lower odds of mild or severe depression. Results are published in JAMA Network Open.

“Getting consistent, regular sleep has wide-ranging effects on our health,” says co-author Susan Redline, MD, MPH, a senior physician in the Brigham’s Division of Sleep and Circadian Disorders, in a release. “Future studies examining bright light therapy should not overlook the role sleep regularity may be playing in influencing mood and depression symptoms.”

Study Methodology and Findings

The study, which was led by first author Danielle A. Wallace, MPH, PhD, also of the Division of Sleep and Circadian Disorders, used data collected from 2011 to 2014 from the National Health and Nutrition Examination Survey. The team investigated whether bright light exposure (at a level generally equivalent to daylight exposure) was associated with depression symptoms, and, if so, whether the sleep regularity index (a measure of the consistency of day-to-day sleep schedules) explained this association. Bright light and sleep regularity were measured using a wrist-worn device.

“We found that greater time spent in bright light was modestly associated with lower depression symptoms and that sleep regularity partly explained this association,” says Wallace in a release. “Higher vitamin D was also associated with greater bright light exposure and greater sleep regularity but not with depression symptoms.”

The authors note that the findings are limited by the cross-sectional nature of the data, and therefore causality cannot be determined. For example, depression symptoms may influence time spent outdoors and bright light exposure. Future research should follow participants over time to evaluate the role of sleep regularity in the relationship between light exposure and mood.