Autoimmune disorder occurrences are decreased by 14+ hours a week of outdoor activities in the sun. Seasonal Affective Disorder is also reduced by getting enough time in the sun. We all need Vitamin D as well.
Healthline recommends, "aim to get 10–30 minutes of midday sunlight, several times per week. People with darker skin may need a little more than this. Your exposure time should depend on how sensitive your skin is to sunlight. Just make sure not to burn."
So, do you get enough sun in winter?
Most Helpful Opinions