Here are some studies about vitamin D, bright light, and seasonal affective disorder:
Conclusion: A study of 101 healthy men found that their serotonin levels were lowest during winter, when there’s less sunlight, but “rose rapidly” as the amount of bright sunlight increased.
Conclusion: This random double-blind placebo-controlled trial found vitamin D3 supplementation to be significantly effective for alleviating depression from seasonal affective disorder.
Conclusion: In a randomized control trial experiment involving 96 seasonal affective disorder patients, three weeks of bright light therapy proved to have an antidepressant effect on winter depression sufferers. (You know… like SUNLIGHT)
Conclusion: In this particular randomized control trial, high-dose vitamin D therapy proved to be an effective treatment for alleviating depressive symptoms of seasonal affective disorder. Could winter vitamin D-deficiency resulting from reduced UVB solar radiation be playing a role in seasonal affective disorder?
Conclusion: Morning bright light therapy is more effective as an antidepressant compared to evening bright light therapy. (Yep, the sun rises in the morning.)
Conclusion: This double-blind study compared the effects after 1 week of a daily 1-hour morning walk outdoors versus a daily half-hour placebo light treatment (only 2800 lux) in seasonal affective disorder patients. The morning walk proved to be effective for regulating melatonin secretion and reducing cortisol, whereas the placebo light treatment did neither.
Conclusion: A “Seasonal Pattern Assessment Questionnaire” involving questions about experience with winter seasonal affective disorder was mailed to people at four different areas of latitude in the U.S.: New Hampshire, New York, Maryland, and Florida. Rates of winter depression were “significantly higher” in the more northern latitudes and this correlation “applied predominantly to the age groups over 35.” The more northern latitudes would be the areas that receive less sunlight.