Sunlight and Its Bioregulatory Effects on Human Physiology

The earth has been bathing in sunlight for more than 3 billion years. The sun produces an enormous amount of energy including cosmic rays, gamma rays, x-rays, ultraviolet radiation, visible radiation and infrared radiation. Ordinarily, sunlight is broken down into three major components:

(1) visible light, with wavelengths between 0.4 and 0.8 micrometre,

(2) ultraviolet light, with wavelengths shorter than 0.4 micrometre, and

(3) infrared radiation, with wavelengths longer than 0.8 micrometre.

The Electromagnetic Spectrum

Most of the ultraviolet radiation is efficiently absorbed by the stratospheric ozone layer; however, some reaches the earth, and with other penetrating solar rays, is essential to life and optimal health. Although ultraviolet light constitutes only a very small proportion of the total radiation that reaches the earth’s surface, this component is extremely important for our health. It produces vitamin D through the activation of ergosterol and plays a role in several other important bioregulatory processes in the body.

The healing power of the sun and its use in medical treatment (heliotherapy) have roots extending back into antiquity. Many traditional medical systems (Egyptian, Greek, Romans, Chinese, etc.) have recognized sunlight as a therapeutic force for millennia. In the modern era, particularly the first half of the 20th century, heliotherapy was widely used in both Europe and North America.

Cultural Changes Affecting Sunlight Exposure

Over time, clothing became the norm in higher latitudes and then eventually a social attribute in many societies. By the 1600s, peoples in these regions covered their whole body, even in summertime. As the industrial revolution swept across Northern Europe in the early 19th century, physicians began reporting that children living in the inner cities of Glasgow and London were developing skeletal deformities especially prominent in the legs as well as growth retardation. By the turn of the 19th century, it was estimated that more than 90% of children living in the industrial cities throughout Europe had this bone deformity disease known as rickets.1

In 1822, Sniadecki reported that children living in Warsaw were afflicted with rickets whereas children living in the rural areas outside of Warsaw did not develop this bone deformity disease. He concluded, “strong and obvious is the influence of sun on the cure of rickets and the frequent occurrence of the disease in densely populated towns where the streets are narrow and poorly lit”.2

It was inconceivable to the medical community of that time how exposure of the skin to sunlight could have any health consequences on the skeleton or disease. Consequently, this observation was ignored for almost 100 years.

Typical living conditions in the Gorbals in 1912. This region of Glasgow was the most notorious slum in the United Kingdom. (Mitchell Library, Glasgow Life)

Typical living conditions in the Gorbals in 1912. This region of Glasgow was the most notorious slum in the United Kingdom. (Mitchell Library, Glasgow Life)

In 1890, Theobald Palm, a medical missionary, wrote to his colleagues living in India and China where nutrition was extremely poor, asking whether they were seeing children with rickets. They reported that it was a rare condition in these countries. He reasoned that children living in London had better nutrition and better housing conditions, and therefore the only common denominator was that children living in the polluted cities of London and Glasgow were not exposed to adequate sunlight. He encouraged sunbathing as a method to treat and prevent rickets. While Palm’s observations were in some ways anecdotal, they had a potent effect on the development of photobiology.3

About that same time in 1893, Niels Ryberg Finsen published a paper about the effects of light on skin. Finsen suffered from Niemann-Pick disease, and noted that his own sluggishness seemed cured with a regular daily dose of sunlight. This inspired him to investigate the effects of light on living things. Finsen discovered that certain wavelengths of light can generate healing properties and was eventually able to demonstrate its effects on a skin condition called lupus vulgaris. He also showed that solar radiation could help treat smallpox and tuberculosis. Finsen won the Nobel Prize in Physiology in 1903 for his work on phototherapy.4, 5, 6