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    How Light Impacts Our Health

    Human beings are exquisitely sensitive to light. We rely on it to set our internal clocks, to produce vital chemicals, and to regulate mood and perception. The human brain takes its timing cues from sunlight; the largest part of the brain is allocated to visual perception, and nerve clusters use the amount and intensity of light to control our sleeping and waking rhythms. 

    effects-of-light-on-health-3

    The Spectrum of Light 

    Light from the sun appears white to us but when passed through a prism, it breaks up into the brilliant rainbow of colors with which we’re all familiar. Each color corresponds to different wavelengths of light, measured in nanometers (billionths of a meter). Colors toward the blue end of the spectrum are more energetic and thus have more “punch” when they fall on living tissue, such as our skin. 

    Violet light is the most energetic and shortest wavelength we can see, at about 300 nanometers. Red light is much longer and carries less energy, with a wavelength ranging from 620 to 780 nanometers. Light waves that are shorter and more energetic than violet light belong to the ultraviolet part of the spectrum. They’re very powerful and can affect our health rapidly.  

    How Much Sun Is Too Much? 

    Safe exposure to the sun, as well as excessive exposure to sunlight, are relative terms. How long you can stay in the sun depends on the time of year, your location on the planet, skin tone, local atmospheric conditions, and the UV Index. 

     Checking your local UV index on a weather app or wherever you get your weather forecast lets you know how much sun is too much. 

    The UV index is a number from 0 to 11 and tells you how long you can stay in the sun with no sunscreen or protective clothing without burning. The UV index levels are: 

    • 0-2. The danger from the sun’s UV rays is minimal. Adults may stay in the sun for up to an hour during the UV peak from 10 am to 2 pm.  Even though this is the lowest danger level, still consider using an SPF sunscreen of at least 30. If you’re going to be in the water, snow, or other highly reflective surfaces, wear UV-blocking sunglasses. 
    • 3-5. This is a low level of danger, requiring SPF 30+ sunscreen and UV-blocking glasses if you’re in the sun longer than 20 minutes. Fair-skinned people risk burning after 20 minutes with a UV index of 3-5 without being protected by sunscreen.  
    • 6-7. At a UV index of 6 or 7, your risk for sun damage without sunscreen or other measures is high. Use sunscreen or protective clothing and UV-blocking glasses. At this level, sun exposure longer than 20 minutes will cause sunburn. At a UV index of 6 and higher, staying in the shade is a good idea. 
    • 8-10. At this very high-risk level, a fair-skinned person without protective clothing or sunscreen will sunburn in less than 10 minutes. Use a broad-spectrum SPF 30+ sunscreen and reapply it every hour if you’re sweating or in the water. 
    • 11 or greater. Unprotected people will get sunburned in 5 minutes or less. If you can do it, avoid going outdoors from 10 am through 2 pm. Wear sunscreen, UV-blocking glasses, and protective clothing if you go outside for any length of time longer than a minute. 

    Note that clothing needs to have a tight weave to offer protection from UV rays, as a loose weave allows the sun’s high-intensity UV radiation to reach the skin.  

    How Does Light Affect Skin Health? 

    Ultraviolet light poses the most serious health risk when we get too much of it. Excessive exposure to ultraviolet radiation (UVR) leads to prematurely aged skin, skin cancer, damaged retinas, and many other health issues.  

    Ultraviolet radiation is composed of ultraviolet A (UVA), ultraviolet B (UVB), and ultraviolet C (UVC). UVC is largely absorbed by the ozone layer in the upper atmosphere, which is fortunate because UVC is extremely potent. UVB is heavily absorbed by the top layer of our skin tissue and is responsible for sunburns and the development of some skin cancers, including malignant melanoma.  

    UVA makes up 95 percent of the sunlight that reaches us. It’s UVA that causes skin to look aged. It penetrates all the way through skin tissues and causes wrinkles, sunspots, and loose skin. The appearance of aging is caused by these powerful waves of radiation breaking down bundles of a protein called collagen. When collagen, the framework of the skin, is weakened, the skin loses its suppleness, leading to sagging, wrinkles, and fine lines. 

    These rays are so powerful they can also damage cells on a molecular level, by affecting the DNA of skin cells. Even slight damage to the cell’s biological code impairs the skin’s ability to heal and reproduce correctly. This genetic damage can do much worse than causing the skin to appear aged. UVA over-exposure can lead to some forms of skin cancer. Chronic exposure to UVR may also cause non-malignant skin disorders like actinic keratosis. 

    Seeing the Light 

    Light enables us to see and perform visual tasks, but chronic overexposure to sunlight may lead to significant impairments in our vision. “Chronic overexposure” for our purposes refers to daily exposure to a UV index of 5 or greater without UV-blocking lenses. It usually takes years for this level of overexposure to cause eye disease, but when it happens, the results can be severe. 

    The following conditions are a few of the most common: 

    • Cataracts. A cataract is a cloudy area on the eye’s lens. Chronic exposure to intense sunlight is a common cause of cataracts. 
    • Photoretinitis. Looking directly at the sun can damage the retina’s ability to absorb light.  
    • Pterygia and pinguecula (Surfer’s Eye). Pterygia are fleshy triangular growths on the surface of the eye that may interfere with vision. Pinguecula is a small mass that develops next to the cornea. They’re not cancerous and are easily removed.  
    • Ocular Melanoma. Ocular melanoma is the most common eye cancer and is caused by unprotected exposure to excessive amounts of sunlight over a period of years. 

    Vitamin Production 

    We rely on sunlight to get vitamin D naturally. When the skin is exposed to UVB, chemical reactions produce cholecalciferol, also known as vitamin D3. D3 is essential for our bodies to use calcium correctly. The immune system also relies on a plentiful supply of vitamin D. 

    Depending on the time of year, people need 10 to 30 minutes of exposure to the sun three times a week to produce healthy levels of vitamin D. In summer, you need brief exposures, but in winter, longer durations are necessary. 

    The Circadian Rhythm 

    The human tendency to be awake during the day and asleep during the night is called the circadian rhythm  Our circadian rhythm relies on complex tracts and clusters of nerves in the eyes and brain to work correctly, but it’s the amount, intensity, and quality of light that drives our sleeping and waking cycles. The process may seem involved but understanding how light affects the circadian rhythm is essential to understanding how light affects a person’s good health. The process works like this: 

    Light passes through the eye and strikes the retina at the back of the eye. The retina is rich with specialized light-sensitive nerves called photoreceptors. When light strikes these photoreceptors, nerve impulses flow through the optic nerve, which passes through the thalamus, deep inside the brain. The thalamus routes physical sensations and nerve impulses from the senses to the correct destination in the brain. 

    Some of these nerve fibers go to the visual cortex, where information from our eyes is further processed and interpreted. However, one bundle of nerve fibers goes from the thalamus to a tiny cluster of nerve cells called the suprachiasmatic nucleus (SCN), which only has 20,000 neurons. 

    Small but mighty, the SCN is the master time-keeper for every human being. It sets a person’s body clock, which affects everything about a person’s health. The SCN communicates with the pineal gland, stimulating it to release the hormone melatonin, which causes us to feel sleepy, regulates our sleep cycles, and acts as a powerful anti-oxidant. As light levels during the day decline, the SCN signals the pineal gland to turn up melatonin production. 

    Artificial Light from Digital Devices 

    Blue light from our phones and tablets can’t harm the eyes beyond occasional eye strain, but it’s blue light the brain uses to determine how much melatonin to release. Many of us relax at bedtime by using our digital devices to read, text, or watch videos and shows. The light from digital devices tends to have a lot of blue in its spectrum. Even though we don’t perceive this light to be blue, it’s there all the same. 

    All this blue light can trick the brain into behaving as if it’s still daylight or day time. For some people, this overexposure to blue light at bedtime makes going to sleep difficult, leading to insomnia. 

    Works Cited 

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