When health-conscious people apply sunscreen regularly to all exposed skin before heading outside, they may actually be doing more harm than good.
People who suffer from seasonal affective disorder are most likely missing out on the visible part of the sun's light spectrum. That's the part that communicates with the brain via the eye, affecting melatonin and serotonin rhythms. In those cases, exposure to full-spectrum sunlight -- even artificial sunlight -- can help to elevate mood [source: Brody]. That's the basis of phototherapy for depressed patients with SAD. Sunscreen isn't necessarily a problem in that regard.
But a lack of exposure to the ultraviolet part of the spectrum -- the light rays that are blocked by sunblock on the skin -- could be helping to keep the depression cycle in business. If you're never in the sun without UV-blocking sunscreen, your body probably isn't producing enough vitamin D. And research shows that loading up on vitamin-D-producing sun exposure in bright summer months can help maintain high serotonin levels through dark winter months [source: Collinge].
And what if you don't suffer from severe winter depression? Does that mean you should feel fine about getting all of your light exposure from incandescent or fluorescent bulbs? Probably not. Too much exposure to limited-spectrum indoor lighting, and the related under exposure to full-spectrum light, has been connected with problems like fatigue, reduced immune function and possibly fertility issues [source: Brody]. What's more, vitamin D deficiency may increase your risk of getting certain types of cancers, including prostate, colon and, believe it or not, skin [source: USA Today].
So do we throw out everything we know about the evils of sunlight and go bake in the sun to our hearts' content?
As with everything else, moderation is key. First, to help your circadian rhythms stay in tune, get all of the sunscreen-protected sunlight you want, and make sure you open the blinds when you're stuck inside. To keep up your supply of vitamin D, aim for about 15 minutes of unprotected sun exposure three times a week [source: USA Today].
For more information on sunlight, circadian rhythms, happiness and related topics, look over the links on the next page.