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-sleeve shirt can affect results. The feature is available on Apple Watch Series 6 or later models.
You might be thinking that you don’t need a watch to tell if you’re spending time in the sun, but you may need to know if you are spending enough time outside.
The typical person is outside for far fewer hours in the day than was the case generations ago, said Richard Weller, a professor of dermatology at the University of Edinburgh.
“Until the Industrial Revolution, 150 years ago, we have lived our entire evolutionary history outdoors all day, every day,” he said. “What is abnormal is that we now spend our lives inside 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 leisure 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 for skin cancer.
But exposure to daylight has been shown to help boost the body’s production of vitamin D, mitigate digital eyestrain, 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 may also benefit the immune system, tamping down inflammation and lowering the risk of developing autoimmune conditions such as rheumatoid arthritis and psoriasis, said JoAnn E. Manson, chief of preventive medicine at Brigham and Women’s Hospital and a professor of medicine at Harvard Medical School.
Manson co-led a nationwide randomized trial examining the potential benefits of vitamin D supplements. The study found that the vast majority of Americans are getting 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 amount of vitamin D that’s recommended, Manson said.
“It doesn’t take a large amount of sun exposure,” she said. “And, in fact, it’s important not to overdo it.”
There’s emerging evidence that time spent in daylight can also boost your mood, increasing serotonin, Manson said. The potential mood-swaying benefits of light exposure are why some people turn on bright light boxes to treat the lethargy of seasonal affective disorder as the days grow shorter in the winter.
When people spend time in daylight, they feel better, said 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 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 topic has more than 1 million views.
“We are finding more and more data for sunlight having significant systemic health benefits,” Weller said.
Why morning sunlight matters
Stepping outside in the morning, within the first few hours after waking up, is the most important signal you can send to your body’s circadian clock, said Samer Hattar, chief and senior investigator for the section on light and circadian rhythms at the National Institute of Mental Health.
“That’s the time when the light is going to have the biggest impact,” he said. “Your system is going to be more entrained and more aligned to the solar day.”
Sunlight “triggers the timed release of cortisol into the body,” which acts as a “a wake-up signal” throughout the day to stay focused, 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, attenuating the amount of melatonin we have during the day, all of that is related to our light exposure,” said Lisa Ostrin, associate professor and researcher at the University of Houston College of Optometry. She said she’s found some children and adults get less than a half-hour of sunlight a day.
How much sunlight do we need each day?
A person’s skin complexion and the clothes they’re wearing, as well as the place, 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 noon on a summer day, in general, people with lighter skin complexions need to spend only 10 to 15 minutes outdoors. But never let your skin burn or even redden and protect your skin with sunscreen as needed, Holick stressed.
For people with darker skin tones, it can take up to 10 times longer for the skin to get 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 the late afternoon, much of the ultraviolet light from the sun is absorbed by the ozone. In the winter, when one hemisphere tilts away from the sun, it becomes even harder to be exposed to the ultraviolet light.
Holick said tracking your time in the sun is “a good thing, but “all daylight is not 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 that people can reduce the amount of time they spend indoors on nearsighted activities such as using their devices. And studies have found children who spend more time outdoors are at a lower risk of developing myopia, also known as nearsightedness.
The International Myopia Institute recommends children spend 80 to 120 minutes a day outside to reduce the risk of myopia.
Researchers are still trying to determine why time outdoors lowers a child’s risk for myopia, said Elise Harb, pediatric optometrist at the University of California at San Francisco and Berkeley. It’s not clear if it’s because of light exposure or that time outdoors means less time spent on nearsighted tasks.
“To me, the best evidence is that it’s simply the brightness of being outdoors,” said Donald O. Mutti, professor in optometry at Ohio State University, who co-wrote 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 may affect myopia risk, he said.
“The sad news is, for those who are currently nearsighted, time outdoors does not seem to influence what glasses you are going to need,” Mutti said.