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22 Strange Ways the Sun May Affect Your Body

The good, the bad, and the sunny

You know that the sun’s ultraviolet (UV) rays can cause skin cancer and premature aging, and that wearing sunscreen (year-round!) is an important way to protect yourself. But research has shown that sunlight also seems to affect health in a lot of other ways, both positively and negatively.
“One American dies every hour from skin cancer, and the overwhelming majority of these cancers are caused by overexposure to UV light—there’s no question about that,” says Darrell Rigel, MD, clinical professor of dermatology at New York University Langone Medical Center. “But that doesn’t mean you should stay inside all day or that there aren’t plenty of health benefits to being outdoors. You just have to balance everything and use common sense to protect yourself.” Here’s what you need to know so you can make smart choices when you step outside.

Mood

Sunlight triggers the release of the feel-good brain chemicals serotonin, so spending time outdoors has been shown to boost mood and relieve stress. And for some people, not getting enough sun during winter months can even trigger a type of depression known as seasonal-affective disorder. (In rare cases, sunlight can trigger depression.)
Luckily, you can fight the effects of SAD, or even a bad mood, without exposing yourself to harmful UV rays, says Dr. Rigel. “You don’t need ultraviolet light to feel better, you just need visible light,” he says. “Just being in a bright room can help.” Special light boxes, which don’t give off UV rays, may also help.

Vitamin D levels

Sunlight helps the body make vitamin D—an important nutrient for healthy bones, brains, and more. Vitamin D is found naturally in very few food sources, so people need to get it either from sun exposure, supplements, or fortified foods, like milk. (Fatty fish, such as salmon, naturally contain vitamin D.) 
You don’t need much sunlight to get adequate vitamin D, especially if you have pale skin or red hair; for most people, just 5 to 30 minutes twice a week, with your face, arms, legs, or back exposed without sunscreen is enough. But still, many Americans don’t get enough. And because of very real skin-cancer concerns, the American Academy of Dermatology recommends getting your vitamin D from a healthy diet and/or supplements—not from the unprotected sun exposure.

Heart health

Many studies have suggested that Vitamin D deficiency is linked to cardiovascular disease. And in 2013, scientists in Denmark found that sun exposure—even when it also causes skin cancer—may actually protect the heart: When they analyzed more than 4 million medical records, they discovered that people who’d been diagnosed with skin cancer were less likely to have had a heart attack (or to have died from any cause) over the course of the study.
“As skin cancer is a marker of a substantial sun exposure, these results indirectly suggest that sun exposure might have beneficial effects on health,” the study authors wrote. However, they caution that this does not prove a cause-and-effect relationship, and that sunlight’s effects on heart health should be studied more directly.

Strong bones and muscles

In that same Danish study, researchers also found that having a history of skin cancer was linked to a lower risk of hip fractures in people younger than 90. That may be because vitamin D helps the body absorb calcium, which is important for healthy bones.
Vitamin D is also important to muscle health, and people with low levels are more likely to experience muscle cramps and joint pain. Sunscreen blocks the body’s ability to make vitamin D, though, so taking supplements or eating fortified foods are still the safest way to get your fill.

Multiple sclerosis

Getting high levels of vitamin D, either from sun exposure or food, may decrease your risk of developing multiple sclerosis (MS). There is strong evidence that people who live at higher latitudes, and therefore get less exposure to UV rays, have a higher risk of the disease than those who live closer to the equator.
A 2014 study also found that correcting vitamin D deficiency may slow the progression of MS and related brain lesions, but the researchers only looked at dietary supplementation—not access to sunlight.

Pancreatic cancer

Rates of pancreatic cancer—the 12th most common cancer in the world and the seventh most deadly—are highest in countries with the least amount of sunlight, found a 2015 study from the University of California San Diego School of Medicine. 
“If you’re living at a high latitude or in a place with a lot of heavy cloud cover, you can’t make vitamin D most of the year, which results in a higher-than-normal risk of getting pancreatic cancer,” said lead author Cedric F. Garland, DrPH, in a university press release.

Breast cancer

Sunlight exposure may also be associated with lower risk for another common cancer, according to a 2014 study published in the journal Environmental Health Perspectives. Women who reported spending an hour or more outside every day for the last 10 years were less likely to have had breast cancer than those who reported less time in the sun.
As with other studies, the authors suspect that vitamin D production is responsible for the link—although they weren’t able to find proof on a genetic level. The authors were also surprised to find that sunscreen use did not seem to affect cancer risk, but they point out that it could be because SPF is often applied too sparingly or infrequently.

Blood pressure

Rates of hypertension tend to be higher in the winter and in countries farther from the equator, and a 2014 study published in the Journal of Investigative Dermatology provides a possible explanation: Exposure to sunlight causes nitric oxide in the skin to be absorbed into the bloodstream, which can help widen blood vessels and lower the pressure inside them.
“Our results add to an increasing body of evidence suggesting beneficial effects of sunlight above and beyond those afforded by vitamin D,” says study co-author Martin Feelisch, PhD, professor or experimental medicine at the University of Warwick in the U.K. He also adds that spending more time in nature may have psychological effects on stress reduction (which can also help control blood pressure).

Lung health

If you live in an urban area that’s dense with cars or factories, the sunshine may be making the air you breatheeven dirtier. That’s because UV rays can trigger the release of smog-forming chemicals from polluted air and the grime that settles on buildings and outdoor surfaces, according to a study presented at the American Chemical Society’s 2015 annual meeting. Air pollution has been linked to asthma and early mortality, and possibly a higher risk of stroke, heart disease, and other health problems. 
Aside from moving away, you may not be able to do much about this problem. But you can keep track of ozone levels in your community, and avoid unnecessary outdoor activity on days that are rated unhealthy. Exercising outside in the mornings can also help, since smog gets worse throughout the day as pollution mixes with sunlight.

Arthritis

A 2013 study published in the Annals of Rheumatic Diseases found that exposure to ultraviolet B radiation—one of two types of rays (UVA and UVB) found in natural sunlight—may reduce women’s risk of rheumatoid arthritis. Study participants who lived in the sunniest areas of the United States (like Hawaii and Arizona) were 21% less likely to have developed the degenerative disease than those that got the least sun (like Alaska and Oregon).
Interestingly, though, the link was only found among older women. This may be because people have become more careful about sun protection in recent years, and the younger participants likely spent more of their lives wearing sunscreen or actively avoiding the sun.

Digestive health

Another study, published in the journal Gut, looked at that same group of women and their levels of inflammatory bowel disease. It found a similar pattern: Participants living in sunny, southern parts of the country were less likely to develop Crohn’s disease and ulcerative colitis than those in northern, less sunny states.
The authors believe that vitamin D levels play a role in the body’s immune response, and its regulation of inflammation in the body.

Allergies

Children who live in low-sunlight areas are more likely to be allergic to eggs and peanuts than those who get plenty of rays, according to a 2012 Australian study. Other research has found a link between low vitamin D levels and various types of allergies (both food and non-food), as well as asthma 
In a 2012 review of previous studies, the authors note that modern-day lifestyle patterns like “indoor occupation, ‘screen time,’ and active sun avoidance” leave people susceptible to inadequate vitamin D levels—”a factor that may be playing a role in the current allergy epidemic,” they wrote.

Eczema and psoriasis

The same Australian study that found a link between food allergies and latitude also found that kids in the south of the country (who get less sunlight) were twice as likely to develop eczema as those in the north. And, in fact, both eczema and psoriasis—another itchy-skin disease—are sometimes treated with UV light, through a process called phototherapy.
However, more sunlight is not necessarily a cure for either condition. According to the UK-based National Eczema Foundation, sun exposure can improve some people’s eczema, but it can exacerbate others’. And skin damaged caused by sunburn (or tanning beds) can make psoriasis worse, warns the National Psoriasis Foundation.

Sleep

Exposure to light during the day—and darkness at night—can help you maintain a healthy circadian rhythm, so you feel alert during your waking hours and tired at bedtime. That’s why it’s a good idea to open the curtains as soon as you wake up in the morning, and to avoid too much artificial light once the sun goes down.
People with irregular schedules, like shift workers, can miss out on daytime sun exposure and often have trouble sleeping. Shift work has also been shown to affect metabolism, and has been linked to health problems including diabetes, memory decline, and heart disease.

Eye health

Sunglasses aren’t just a fashion statement; they also protect against harmful UV rays that can damage your vision. Direct exposure to sunlight can damage your retinas, and too much sun over time can lead to macular degeneration, glaucoma, and cataracts. It’s not all bad news, though: A 2009 study from the New England College of Optometry found that kids who spend more time outdoors are less likely to develop myopia, also known as nearsightedness. 
To protect your peepers, wear sunglasses with 99 to 100% UV protection, even when it’s cloudy outside. (Wearing shades without UV protection is even more dangerous than wearing nothing at all, since your pupils will dilate behind dark lenses, letting in more light.)

Headaches

For people who get migraines, sun glare or flickering lights—like sunlight filtering through the trees as the wind blows—can trigger headaches. Avoiding bright light can help; so can taking preventative medicine if you know you’re at risk and you’ll be spending time outdoors.
Sun-induced headaches can also be a sign of heat exhaustion or dehydration, especially if you’ve been outside for a while on a hot summer day. Staying hydrated, seeking out shade, and keeping cool can help you stay safe and pain-free.

Pregnancy and fertility

Women who have low vitamin D levels during pregnancy are more likely to experience complications like gestational diabetes and preeclampsia, and to develop multiple sclerosis in the years after giving birth. Their babies are also more likely to be born underweight, and to carry their own increased risk for multiple sclerosis.
And if a couple is trying to get pregnant in the first place, the sunshine vitamin may help with that, too. A 2008 Australian study found that when D-deficient men improved their levels through sun exposure and supplements, they decreased their levels of “sperm fragmentation” and had more success conceiving with their partners.
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Melanoma

While sun exposure is the clear cause of the most common types of skin cancer—basal cell carcinoma and squamous carcinoma—it also plays a role in the more rare and dangerous melanoma. Melanomas can develop on parts of the body that rarely see the sun, like the mouth, and in the past, the role of sunlight was unclear. Research now shows that frequent sunburns in childhood and the use of tanning beds can increase the risk of melanoma. The use of tanning beds before age 30 ups the lifetime melanoma risk by 75%.

Rickets

In the past, children with poor diets who did not get adequate exposure to sunlight often developed a bone deformity known as rickets. Much less common in modern times thanks to vitamin-D fortified foods, ricketsbrings on symptoms like delayed growth, bowed legs, muscle weakness, and pain in the back, pelvis, and legs. Children with dark skin (which blocks sunlight), who live in countries far from the equator (and thus get less sun), are exclusively breastfed (breast milk lacks vitamin D), or who take medication or have a condition that interferes with vitamin D absorption can be at risk.

Jaundice

Babies who are born with jaundice, a condition that can lead to brain damage or even death, can be treated with sunlight, according to a 2015 Stanford University study. Jaundice occurs when newborns haven’t yet developed an enzyme needed to get rid of waste products in the blood stream, and it can cause a yellowish tint to their skin and eyes.
Not just any sunlight should be used to treat jaundiced babies, however. This study specifically looked at sunlight that passed through special canopies, which filtered out harmful UV rays. The babies were also monitored for overheating and sunburn throughout their treatment.

Sun allergy

It’s extremely rare, but for some people, spending just a few minutes in the sun can cause itching, burning, and debilitating pain. This condition, called erythropoietic protoporphyria and sometimes described as an “allergy to the sun,” affects about 500 to 600 people in the United States.
Luckily, a new drug currently in development has shown promise in treating people with this disorder. The medication, called afamelanotide, works by stimulating the production of melanin—the pigment that gives skin color, and that protects against sun damage in healthy people.

Skin damage (hours later!)

Lastly, it’s a well known fact that sunlight causes skin damage—but did you know it continues to destroy cells for hours after exposure, even once you’re indoors or in the dark? That’s what Yale University researchers found when they studied the effects of UV light on both human and mouse skin-pigment cells. 
Dr. Rigel says that skin damage is an obvious sign you’re spending too much time in the sun without protection. “Skin cancer is the extreme of skin aging, but before that you can certainly get lines, wrinkles, freckles, broken capillaries—all signs of damage,” says Dr. Rigel. “You’re aging your skin faster every time you go outdoors without sunscreen.”

What’s the solution?

While skin cancer (not to mention premature skin aging) is a very real danger, some experts—like Feelisch—say that avoiding the sun entirely may be an even bigger one. “Balancing beneficial with harmful effects of anything starts with the realization that there may be more to the beneficial side than previously considered,” he says. “Considering how humans evolved—being exposed to natural light for almost half of the day—sunlight can’t possibly be that bad, can it?”
Feelisch believes that increased rates of skin cancer are likely due to people “swinging between extremes” of spending most time indoors, then getting way too much sun on vacations and holidays. His suggestion for balancing it all? “Get as much sunshine as you can regularly, while avoiding getting a sunburn.”
Dr. Rigel, on the other hand, takes a cautious approach—an approach that’s echoed by the American Academy of Dermatology and the World Health Organization. “You can safely get vitamin D from supplements, so why expose yourself to a known carcinogen when you don’t have to?” he asks. His advice is to play it safe: “Wear protective clothing, avoid the mid-day sun when UV rays are strongest, and regularly use sunscreen. Those three things will lower your risk of skin cancer, and allow you to enjoy your time outside.”

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