Vitamin D: The effects of too much and too little sunlight on human health

The Sun

The Sun is the star at the center of the Solar System. The most important source of energy for life on Earth, this near-perfect sphere of hot plasma is 150 million km (93 million mi) away from us. Yet on some days, it looks and feels as if it's just hanging in the sky... and that can be a good thing, and a bad thing.

Vitamin D

The UV in sunlight is a principal source of vitamin D. When your skin is exposed to sunlight, it makes vitamin D from cholesterol. Vitamin D, known as the sunshine vitamin, has many roles in the body and is essential for optimal health. 

Calcium and phosphate levels

Vitamin D helps regulate the amount of calcium and phosphate in the body. These factors are vital for maintaining healthy bones and muscle. Vitamin D may also protect against respiratory infections, cognitive decline, dementia, and more.

Mood boost

Another benefit of UV exposure is improvement in mood. The Sun's rays provide warmth and light that enhance your general feeling of well-being. Sunlight can also stimulate blood circulation, which boosts energy levels.  

Sleep-wake cycle

Sun exposure is associated with the timing of melatonin synthesis. Melatonin is a hormone that regulates the sleep–wake cycle. Sunlight is also responsible for the maintenance of normal and robust circadian rhythms—a natural, internal process that regulates the sleep-wake cycle and repeats roughly every 24 hours.

Reduced risk of seasonal affective disorder

Regular exposure to Sun can reduce the risk of seasonal affective disorder (SAD).

"Winter blues"

Sometimes referred to as the "winter blues," SAD has been linked to a biochemical imbalance in the brain prompted by shorter daylight hours and less sunlight in winter. Symptoms–which include sleeping too much, having little to no energy, and overeating–can be distressing and overwhelming, and can interfere with daily functioning. The condition in the summer can include heightened anxiety.

SAD treatment

Treatments for classic (winter-based) seasonal affective disorder include light therapy (pictured), which involves sitting in front of a light therapy box that emits very bright light (and filters out harmful UV rays). It usually requires 20 minutes or more a day, typically first thing in the morning, during the winter.

Dangers of long-term exposure to sunlight

Long-term sunlight exposure is known to be associated with a number of negative health issues.

Melanoma

The rarest but most dangerous type of skin cancer is melanoma. The primary cause of melanoma is UV exposure in those with low levels of the skin pigment melanin. Those with many moles, a history of affected family members, and poor immune function are at greater risk.

Skin aging

Overexposure to the Sun is associated with the development of skin aging, a process technically termed photoaging. Attributed to continuous, long-term exposure to UV radiation, early symptoms of photoaging include the appearance of wrinkles around the eyes, mouth, and forehead, and loss of color and fullness in lips.

Cataracts

While cataracts are most commonly due to aging, prolonged exposure to sunlight, among other causes, can also lead to a clouding of the eye lens and a subsequent decrease in vision.

Sunburn

The most obvious result of overexposure to UV radiation from the Sun is sunburn. Signs and symptoms include pinkness or redness of skin, and pain, tenderness, and itching. More prolonged exposure can cause the skin to swell and blister. Headache, fever, nausea, and fatigue can set in if the sunburn is severe.

Snow blindness

Bright sunlight can induce snow blindness, or photokeratitis. This is a painful eye condition caused by exposure of insufficiently protected eyes to UV rays from either intense sunlight or artificial sources, including the electric arc of a welding tool. The reflection off snow and ice serves to increase the intensity of the UV rays.

Heart disease

Research suggests that vitamin D deficiencies in men caused by sunshine deficit make them twice as likely to develop heart disease.

Least sunniest place on Earth

UN climate records indicate that Tórshavn, the capital and largest city of the Faroe Islands, gets an annual average of just 840 hours a year of sunshine, making it the least sunniest destination on the planet.