Research has shown that messing up your melatonin levels over the longterm makes cancer much more likely (page 250 of Fitness For Geeks), which is one of the many reasons that exposing yourself to real sunlight during the week is not a trivial matter.
Yet we do the opposite (shrink and cover up from the sun), and we're often advised to completely avoid the sun. Sun worship is different than getting healthy doses of the sun, and the conventional wisdom about the sun reminds me of the "avoid eating fat" nonsense (nonsense because of what it ends up being replaced with: sugar in all of its manifestations, plus you become starved of fat-soluble vitamins).
The newest research about the sun and its benefits continues to contradict the notion of slathering your body with sun-screen all the time.
Is sunlight protective against Alzheimer's?
One study, in a Neurology journal, found that non-melanoma skin cancer (NMSC) in people was strongly correlated with not getting Alzheimer's disease, and vice versa (people with Alzheimer's tended to not have non-melanoma skin cancer).
Alzheimer's is the sky-rocketing disease that forty percent or more of us will get (if we don't make changes in our lifestyles), and it is much worse than non-melanoma skin cancer, which mostly involves getting a dot on your skin frozen off with a liquid nitrogen wand.
It was a small study, and did not prove a cause-and-effect relationship concerning the sun being protective against Alzheimer's, but the correlation it found was "astonishingly strong," according to the study authors.
Perhaps it's as simple as: people are outdoor exercising a lot (a lifestyle that is relatively protective against Alzheimer's), and may tend to get NMSC, but possibly further studies will tease out the protective element of sunlight exposure.
I'm sure you've heard the Paleo argument that we evolved for hundreds of thousands of years in sunlight, and that's why we're somewhat like plants, we bio-generate vitamin D from the sun. Thousands of human genes are linked with the vitamin D receptor, and this biochemical is more of a prehormone than a "simple vitamin."
Sun exposure can lower blood pressure
Sunlight releases nitric oxide in the skin that lowers blood pressure, according to another recent study.
The study authors (remember, these were dermatologists or skin doctors) concluded that staying out of the sun could be more risky than getting some sun during the day.
This is because high blood pressure, which depending on the extent, may lead to stroke or heart attacks, is a symptom associated with far more deaths than skin cancer.
Disease incidence, including of melanoma, tends to increase with geological latitude (meaning the lower the intensity of ambient sunlight (at higher latitudes, toward the Arctic), the higher the incidence of diseases like cancer).
When I was on Kauai (first trip ever to that beautiful island) I did a few searches in order to, admittedly, confirm my pro-sun bias.
Hawaii, are they miserable with all that sun?
Hawaii, by far, is closer to the equator and has more direct, stronger sunlight than other U.S. states. Yet, it has the highest life expectancy among U.S. states (if it were a country, it would have among the highest life expectancies in the world).
Hawaii has about one-third the rate of melanoma of Vermont, where I live (and believe me, we go through long "sun challenged" stretches here, otherwise known as cloudy days).
Shouldn't people in Hawaii be dropping dead of sunlight all over the place (to be fair, there are some sunny, high mountain places like Utah that have high melanoma rates)? They aren't–they're out hiking, surfing, and gardening.
Melanoma is undoubtedly caused by additional factors–including vitamin D deficiency.
Hawaii residents are also the perennial winners of the "happiest citizens" in the U.S. as compiled by sociologists who do human happiness studies (Vermont was fifth!), which might also say something else about copious sunlight.
Trust your own instincts
A friend of mine, who's Italian by origin and gets as tan as a Naples beachcomber (or olive harvester) in the summer, listened to my opinions and pointed out that he "feels so much consistently better" in the summer, and his mood lifts in the winter when he skis in the sun.
Not everything has to be based on the conclusions of valid studies; if being in the sun makes you consistently happier and sick less often, then something legit is probably taking place physiologically.
How much sun is good for you?
That's a debatable question that depends on the person, skin type, etc. but I don't think it's a stretch to say that maintaining a light tan (for caucasians like me) is healthy, and up to 45 minutes per day full body exposure outside of the 10am to 2pm window is fine, as long as you avoid any burning.
I let my son, who runs around in various camps all summer in the U.S. northeast, tan naturally without any sunblock. It's quite amazing how few times he gets sick during the Summer (and he has a lousy diet, too much sugar :), compared with the colder months, relatively starved of sunlight.
The first two parts of this series:
(1) The Sun Is Good For You, Imagine That: Part 1
(2) The Sun Is Good For You: Part II, Vitamin D and The Vitamin A Synergy Issue