By | November 24, 2023
How much sun do you get?  Now your watch can keep track.

Your smartwatch tracks your steps and sleep. Now the makers of the Apple Watch want to track your sunshine.

Apple’s Time in Daylight feature uses the watch’s ambient light sensor, as well as its GPS and motion sensors, to detect whether a person is outside. Clouds, shade and even a long-sleeved shirt can affect the results. The feature is available on Apple Watch Series 6 or later models.

You might think you don’t need a watch to see if you’re spending time in the sun, but you might need to know if you’re spending enough time outdoors.

The typical person is outside for far fewer hours a day than was the case generations ago, says Richard Weller, professor of dermatology at the University of Edinburgh.

“Up until the Industrial Revolution, 150 years ago, we lived our entire evolutionary history outdoors all day, every day,” he said. “What is abnormal is that we now spend our lives indoors and briefly pop outside.”

Fewer than 1 in 5 Americans surveyed in the 2021 General Social Survey said they go outside every day to enjoy recreational activities such as hiking, swimming, skiing or just relaxing.

Why do we need sunlight?

Many people worry about the risks of getting too much sun, which can cause sunburn, premature aging of the skin, wrinkles and a higher risk of skin cancer.

But exposure to daylight has been shown to help increase the body’s production of vitamin D, relieve digital eye strain, regulate a person’s circadian rhythm, and increase alertness and mood.

Vitamin D helps the body absorb calcium to build and maintain strong bones. Vitamin D can also benefit the immune system, suppress inflammation and reduce the risk of developing autoimmune conditions such as rheumatoid arthritis and psoriasis, says JoAnn E. Manson, director of preventive medicine at Brigham and Women’s Hospital and professor of medicine at Harvard Medical School.

Manson led a nationwide randomized trial investigating the potential benefits of vitamin D supplementation. The study found that the vast majority of Americans get the vitamin D they need from their diet and the sun. Even 15 minutes of sun exposure without sunscreen two to three times a week will help your body produce the recommended amount of vitamin D, Manson said.

“It doesn’t take a huge amount of sun exposure,” she said. “And in fact, it’s important not to overdo it.”

There’s new evidence that time in daylight can also boost your mood, which boosts serotonin, Manson said. The potential mood-altering benefits of light exposure are why some people turn on bright light boxes to treat the lethargy of seasonal affective disorder when the days get shorter in winter.

When people spend time in daylight, they feel better, says Michael Holick, professor of medicine at the Chobanian & Avedisian School of Medicine at Boston University. And it’s not just because they’re taking a break from work. The sun’s ultraviolet radiation can also encourage the production of endorphins, Holick said.

An observational study in Sweden found that avoiding sun exposure can increase the risk of death. This is at least in part because exposure to sunlight increases the release of nitric oxide in the body, dilating blood vessels, potentially lowering a person’s blood pressure, Weller said. His TED Talk on the subject has more than 1 million views.

“We’re finding more and more data for sunlight having significant systemic health benefits,” Weller said.

Why morning sunlight is important

Getting out in the morning, within the first few hours of waking, is the most important signal you can send to your body’s circadian clock, says Samer Hattar, director and senior investigator of the Section on Light and Circadian Rhythms at the National Institute of Mental Health.

“That’s the time when the light will have the biggest impact,” he said. “Your system will become more withdrawn and more attuned to the solar day.”

Sunlight “triggers the timed release of cortisol in the body,” which serves as a “wake-up signal” throughout the day to maintain focus, Elizabeth Ko, assistant clinical professor of medicine at the David Geffen School of Medicine and medical director of the UCLA Health Integrative Medicine Collaborative , wrote in an email.

Exposure to daylight helps regulate the sleep-wake cycle of our circadian rhythm.

“In terms of regulating our circadian rhythm, releasing dopamine, dampening the amount of melatonin we have during the day, all of this is related to our light exposure,” says Lisa Ostrin, associate professor and researcher at the University of Houston College of Optometry. She said she has found that some children and adults get less than half an hour of sunlight a day.

How much sunlight do we need each day?

A person’s complexion and the clothes they wear, as well as location, season, weather and even the pollution in the air will affect how much sunlight they need each day, Holick said.

To produce vitamin D, the skin needs to be exposed to a certain spectrum of ultraviolet light.

Around midday on a summer day, people with lighter skin tones generally only need to be outdoors for 10 to 15 minutes. But never let your skin burn or even turn red, and protect your skin with sunscreen when needed, Holick stressed.

For people with darker skin tones, it can take up to 10 times longer for the skin to receive enough ultraviolet light to produce the same amount of vitamin D, Holick said.

Experts recommend spending time in the sun between 10 a.m. and 3 p.m. In the morning and late afternoon, much of the ultraviolet light from the sun is absorbed by ozone. In winter, when one hemisphere is tilted away from the sun, exposure to ultraviolet light becomes even more difficult.

Holick said it’s “a good thing to track your time in the sun, but “not all daylight is the same as it relates to its potential health benefits for a person.”

Sunlight and the eyes

Apple says the company added Time in Daylight so people can reduce the amount of time they spend indoors on nearsighted activities like using their devices. And studies have found that children who spend more time outdoors are at a lower risk of developing myopia, also known as myopia.

The International Myopia Institute recommends that children spend 80 to 120 minutes a day outdoors to reduce the risk of myopia.

Researchers are still trying to determine why spending time outdoors reduces a child’s risk of nearsightedness, said Elise Harb, a pediatric optometrist at the University of California at San Francisco and Berkeley. It is not clear whether this is due to light exposure or that time outdoors means less time spent on myopic tasks.

“To me, the best evidence is that it’s simply the brightness of being outdoors,” said Donald O. Mutti, a professor of optometry at Ohio State University, who co-authored a 2007 study on the link. Light outside is often 10 to 100 times brighter than indoors. The bright light triggers the release of dopamine in the eye that can affect the risk of myopia, he said.

“The sad news is that for those who are currently myopic, time outdoors doesn’t seem to affect what glasses you’ll need,” Mutti said.

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