阳光如何治愈我们？| 今日心理学
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翻译组May Li
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CIRCADIAN RHYTHM
Sunlight Heals and Cures. But How?
It’s not just Vitamin D. Synchronizing circadian clocks is important for health.
KEY POINTS
Sunlight increases the risk of skin cancers, but lessens the risk of dying from them.
Exposure to sunlight extends the lifespan and reduces the risk of heart disease and other cancers.
The classic explanation has been that Vitamin D synthesized in the skin has protective effects.
Synchronizing circadian clocks with blue-rich daylight is as effective, without skin exposure to sunlight.
You have heard all the warnings about too much exposure to sunlight and the risk of developing skin cancers. And that is true. U.S. Navy personnel extensively exposed to sunlight have an eight-fold increase in malignant melanomas. A similar eight-fold increase in malignant melanomas was found in Swedish women with extended exposure to sunlight, even if they used sunscreen ointments. This is because most sunscreen ointments insufficiently protect against ultraviolet radiation skin damage.
But sunshine also has a considerable curative effect. Despite the increased skin cancers, people who are exposed to the most daily sunlight have the longest life expectancy. Those U.S. Navy personnel had a death rate from those skin cancers that was three times lower than expected, and they had a 44 percent lower rate of deaths from all other forms of cancer.
Similarly, the Swedish women with the most exposure to sunlight live the longest, even though they get the most skin cancer.1 In comparison, women with the lowest exposure to sunlight (i.e., spending the most time indoors) had double the rate of dying during a 20-year study as compared to women with the most exposure to sunlight. The risk of dying from cardiovascular disease was 130 percent greater and the risk of dying from other non-cancer, non-cardiovascular causes was 70 percent greater. The women with the least sun exposure had about a 40 percent increased risk of hypertension and diabetes. Remarkably, even those who develop malignant melanoma had longer relapse-free survival if they continued to sunbathe.
So, what is it about sunlight that promotes longevity and good health?
Vitamin D Hypothesis
The classic explanation has been that sunlight falling on the skin converts cholesterol in the skin into Vitamin D, and those who live indoors most of the time or live in northern cloudy environments with little sun (think Seattle or Helsinki) have a higher risk of Vitamin D insufficiency. Vitamin D insufficiency has been linked to a wide range of disorders that impact lifespan, and some studies of Vitamin D supplements have shown a lower rate of cancer deaths. The problem is that multiple large-scale randomized controlled trials of Vitamin D supplementation have failed to show significant positive impacts on cancer, cardiovascular events, and mortality.2 That’s not to say that you should stop taking Vitamin D supplements, because there are well-documented positive effects on the health of your bones.
Other Skin Radiation Hypotheses
There are undoubtedly many other mechanisms by which sunlight falling on the skin triggers physiological responses. For example, nitric oxide metabolites in the skin react with UV radiation to release nitric oxide, which lowers blood pressure.3 At the other end of the light spectrum, red light shone on the skin lowers blood glucose levels.4
The limitation of these skin radiation hypotheses is that they depend on a significant amount of skin being exposed to sunlight. If you are on a beach on a sunny day in swim trunks or a bikini, then large amounts of Vitamin D and nitric oxide are synthesized.
However, most people do not sunbathe. Most of the time, they wear clothes, so the only skin exposed to sunlight is their face, neck, and hands, which comprises less than 10 percent of the body's skin surface. Given the vagaries of weather, cloud cover, and being in the shade, the benefits of direct skin radiation may be limited.
Circadian Blue Hypothesis
Outdoor daylight and/or sunlight are clearly beneficial to the health of the circadian timing system because of the intensity and spectral composition of the light falling on the eyes. Even on a cloudy day, the amount of blue wavelength light entering the eyes is more than sufficient to provide a strong synchronizing cue to the circadian clock system. This helps to prevent circadian disruption and thus reduce the risk of multiple diseases, increasing lifespan.
So simply going outside each day, without needing to sunbathe or expose large areas of skin, should be part of everyone’s daily routine. As an added plus, if you do it in the morning, before the midday sun, you get the strongest circadian boost and also minimize the risk of skin cancer.
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愿景
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