Many public messages have focused on the dangers of too much sun exposure such as aging, skin cancer and DNA damage. However, sunlight has many beneficial effects on human health.

Given the importance of sunlight for life on this planet, all lifeforms including humans have evolved to use the power of the sun to their advantage.

 

• Sunlight and vitamin D:

It is the best well known benefit, vitamin D3 is synthesised in the skin through a photosynthetic reaction by exposure to UVB radiation. Vitamin D accumulates in cells of the intestines where it enhances calcium and phosphorus absorption and controlling the flow of calcium into and out of bones. Adequate vitamin D production is vital for healthy bones. In children, vitamin D deficiency causes rickets, a disease that retards growth and causes skeletal deformities such as bowed legs. Low vitamin D causes and worsens osteoporosis and bone pain from osteomalacia.

Vitamin D is a hormone, so the body relies on it for overall health. Laboratory studies show that vitamin D can reduce cancer cell growth, help control infection and reduce inflammation. It was shown to reduce the risks of developing Rheumatoid Arthritis, diabetes, and heart disease. It strengthens the immune system and there is evidence that it helps to reduce the severity and duration of Covid 19 infection.

 

• Sun exposure and heart health:

Researchers have noted a link between sun exposure and lower blood pressure levels with reduced death rates from cardiovascular issues. It is suggested that sunlight triggers the skin to release nitric oxide, which causes arteries to dilate, lowering blood pressure and may reduce the impact of metabolic syndrome.

 

• Sunlight sets circadian rhythm:

Light plays a central role in regulating the body’s internal clock that signals when to be alert and when to rest. It is most beneficial if getting sunlight exposure into the eyes in the morning just after waking. A good circadian rhythm is important for regulation of sleep wake cycles, hormone release, body temperature and other important bodily functions. Getting enough sunlight during daytime is essential for good sleep and increases night-time melatonin levels.

 

• Sunlight and mental health:

Exposure to sunlight increases the brain’s release of serotonin. Serotonin is associated with boosting mood and helping a person feel calm and focused. Low levels of serotonin are associated with higher risk of major depression with seasonal pattern (seasonal affective disorder or SAD) that is triggered by changing seasons. The light-induced effects of serotonin are triggered by sunlight that goes in through the eye.

One study found chronic sun deprivation leads to cognitive impairment. Even brief exposure to sunlight substantially increases alertness and thinking ability.

Red to infrared light therapy (wavelengths 600-1070nm) and, light in near infrared range is capable of arresting neuronal death. This therapy is being explored for Alzheimer’s and Parkinson’s disease. In some patients with Parkinson’s, light exposure improved mood, social activity, motor function and in some cases, reduced medication for Dopamine replacement.

 

• May protect against cancer:

Although skin cancer has been associated with too much UV radiation, a number of other cancers could result from too little sun.

For example, those who live in places with less sun (in high latitudes) are at higher risks of dying from breast, ovarian, colon, pancreas, prostate, and other cancers. Ironically, high sun exposure increases survival rates in patients with early-stage melanoma.

 

• Support longevity:

In a Swedish study, there was a two-fold increase in mortality among those who avoided sun exposure. Non smokers who avoided sun exposure had a life expectancy similar to smokers in the highest sun exposure group indicating that sun avoidance may be detrimental as smoking for health.

 

• Helps to prevent diabetes:

Vitamin D deficiency has been associated with insulin resistance, type 1 and type 2 diabetes and metabolic syndrome. Vitamin D receptors are present in the pancreas, increasing insulin secretion and sensitivity.  UV exposure lowered weight gain, glucose intolerance and Insulin resistance in mice fed with a high fat diet. The same benefits were not seen with vitamin D supplements, may be partly due to sunlight stimulates the production of nitric oxide.

 

• Prevention of myopia in children:

Studies in Taiwanese children showed that myopia progression can be diminished by increasing outdoor time. Exposure to outdoor light led to less myopic shift, less axial elongation, and a 54% lower risk of rapid myopia progression. This study supports the role of light exposure in childhood myopia prevention. Children needs to be exposed approximately about 2 hours of daylight per day. When outside, we can focus our eyes on objects in the far distance.

 

• Sunlight and the immune system:

The risk of immune mediated diseases such as Multiple sclerosis, rheumatoid arthritis, Ulcerative colitis, and lupus was shown to be highest with mothers who had low UVB exposure and low vitamin D status in the 2^nd and 3^rd trimester of pregnancy.  Multiple sclerosis and Rheumatoid arthritis incidence is higher in regions at a higher latitude and low UVB exposure.

Multiple sclerosis incidence increased 8 times in Iran in the past 20 years due to lack of vitamin D and sun exposure in women.

Vitamin D is a crucial immunomodulatory factor as it strengthens the immune system but also prevents the over-activation that leads to autoimmunity. In animal models, lack of vitamin D worsened Irritable Bowel Disease, Rheumatoid Arthritis, Multiple Sclerosis, and other autoimmune conditions. Vitamin D supplementation in humans has shown mixed results for autoimmunity. This indicates a more complex protection mechanism from sun exposure including the immunosuppressive effects of UV light. The UVB light boosts regulatory T cells which prevents excessive inflammation and autoimmune response.

 

• Natural painkillers:

Post-surgery exposure to sunlight decreased stress and pain thus minimising the need for pain killers by 21% due to its ability to increase beta-endorphins in the skin in one study.

 

• Light used in therapy:

The spectrum of sunlight reaching the earth consists of:

a- UltravioletB (UVB)- wavelengths 280-315nm. UVB can enhance skin ageing, damage DNA causing sunburn and promotes skin cancer. But UVB is needed to produce vitamin D3.

b-Ultraviolet A (UVA- 315-400nm). UVA was considered to be less damaging to DNA and it is used in cosmetic artificial skin tanning and PUVA therapy (psoralen and UVA) for psoriasis. However, UVA is now known to cause significant damage to DNA via indirect routes (formation of free radicals and reactive oxygen species) and can cause cancer.

c- Visible range 380-700nm.

Blue light therapy is approved by FDA to treat acne, it kills bacteria Propionibacterium acnes on the skin and is also used to treat sun damage or premalignant skin cancer growth.

Green light therapy is claimed to reduce excess melanin and reduce dark spots, pigmentation and treating erythema by slowly causing the small blood vessels to close and disappear.

Bright white light therapy (4200 lux) 30 min exposure in the morning for dementia patients can improve insomnia as light stimulates the production of melatonin.

d- Infrared range 700nm-1,000,000nm (1mm)

Red light and near infrared light therapy is now widely used and researched in the treatment of various conditions:

• Far infrared (FIR) or radiant heat is part of the natural light spectrum that warms the objects without affecting the temperature of the surrounding free air. FIR transforms light to heat energy, dilating blood capillaries to increase blood flow, speeds cellular metabolic rates by stimulating the activity of mitochondria and promotes healing of injured tissues… It is used by physical and massage therapists for treatment of arthritis, joint pain, stiff muscles, and injuries to tendons and ligaments.
• Red light therapy used red light and near infra-red region because this wavelength range best penetrates through tissues about 5mm depth. It is a popular option to treat acne, hair growth, wrinkles as it may stimulate rapid production of cells that secrete collagen. There are many unproven claims that RLT that can treat every health condition including activate cancer treatment…
• The research exploring light therapy is promising that bright light and photobiomodulation could benefit in patients with dementia. Scientists in UK used a specially designed helmet emitting infrared light for dementia patients and it was shown to improve memory, processing skills and motor function. But the research is still in its infancy and there is not enough evidence to recommend RLT as an intervention in dementia.

 

In conclusion, there is a need to be a balance between the risks of having too much sunlight or too little. There is a general consensus that sunburn should always be avoided. There is no set rule for the ideal duration of sun exposure, as it depends on many factors such as the skin types (fair or dark skin), surface area of exposed skin and the intensity of sunlight (time of the day). Epidemiological data shows that people who have more exposure to sunlight have lower blood pressure and cardiovascular related mortality. While excess sunlight is a risk factor for skin cancer, sun avoidance may carry more harm than benefit for overall good health.

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