sun and the body

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sun and the body The sun has exercised a powerful influence on the physical and mental lives of human beings. The sun emits visible light, heat, ultraviolet rays, radio waves, and X-rays. Ultraviolet light affects the human body in a number of ways. One of the greatest health benefits of ultraviolet light is the production of vitamin D, which is essential to calcium metabolism and the formation of bone. High energy ultraviolet light enters the skin and causes the photochemical conversion of 7-dehydrocholesterol to previtamin D3, which at body temperature undergoes isomerization to vitamin D3. When the skin is exposed to excessive sunlight, previtamin D3 is changed into two biologically inert substances, lumisterol and tachysterol, which prevent the synthesis of excessive amounts of D3. The production of vitamin D is affected by time of day, season, and latitude. A century ago, the lack of sunlight in industrial cities, and hence, of vitamin D, caused widespread rickets, characterized by bowleggedness. There is evidence that breast-fed babies require exposure to ultraviolet light — perhaps only 30 minutes a week — in order to acquire adequate vitamin D. Studies also reveal that mobility-impaired geriatric patients have only about one-quarter of the serum vitamin D of healthy middle-aged persons. Dietary changes and increased sunlight may help solve this problem. Vitamin D can also be taken in various vitamin supplements.

Ultraviolet light has several therapeutic effects. In combination with drug therapies, it is useful in treating skin diseases, such as psoriasis, herpes, and eczema. ‘Phototherapy’ — exposure not to ultraviolet but to light from the blue end of the spectrum — is used for jaundice in new-born babies, caused by the immature liver's inability to rid the body of bilirubin, which, if it accumulates excessively, can cause brain damage.

Another positive effect of the sun is psychological. In the early 1980s a report appeared of a woman whose depression increased when she went north in the winter and disappeared when she visited Jamaica. Her condition later acquired the name Seasonal Affective Disorder (SAD). An accepted treatment for this condition is exposure to a full-spectrum light of at least 2500 lux — 5–10 times as strong as standard indoor lighting. By comparison, sunlight gives 10 000 lux on a cloudy day, and 80 000 lux (at the Equator) on a sunny day.

The human body apparently evolved a daily rhythm in response to the length of daylight. Sunlight suppresses the secretion of prolactin, which aids in rest; melatonin, which affects mood and subjective energy levels; and growth hormone, which is needed for bodily construction and repair. When male subjects were put on a 10-hour photoperiod, they typically experienced a 2-hour period of non-anxious wakefulness in the middle of their sleeping period, and prolactin was found to be elevated for 14 hours. Those on a 16-hour, modern period, experienced less melatonin secretion than other test subjects, while growth hormone levels doubled. Women seem to be more sensitive to seasonal changes in length of day, perhaps explaining why they are more susceptible to SAD. Photoperiodism also seems to be involved in the production of the neurotransmitter, serotonin, which controls the appetite for carbohydrates. People affected by SAD crave carbohydrates during the winter.

Sunlight can also harm the human body. Excessive exposure of the unprotected skin results in erythema (sunburn). With one Minimum Erythemal Dose (MED) the skin turns pink and starts to produce melanin. With 5–10 MED an excruciating sunburn results after 4–14 hours. Certain medications, including tetracyclines and estrogens, increase the skin's susceptibility to sunburn. The body has ways to protect itself from skin damage from sunlight. Repeated exposure to the sun creates a tan, as melanin accumulates close to the surface of the skin. A deep tan can filter out 95% of the sun's rays. However, after only 2–3 minutes of exposure to the sun, skin damage begins. Two main structural proteins of the skin, collagen and elastin, begin to break down, ultimately resulting in wrinkles. The skin has the ability to repair itself, but repeated and prolonged exposure to the sun damages the skin permanently. Prolonged exposure to the hot sun or any other heat source may cause heat exhaustion or the more serious heatstroke or sunstroke.

By far the most serious ill effect of the sun is skin cancer. Skin exposed chronically to the ultraviolet light of the sun shows a tenfold increase in mutations of a gene called P53. Sunlight further causes the cells containing the mutated cells to spread, where they copy themselves. This mutated gene has been linked with basal cell and squamous cell skin cancers. The most serious form of skin cancer, melanoma, has also been linked with exposure to the sun. Even one or two blistering sunburns in childhood have been associated with an increase in the incidence of melanoma.

The sun has profoundly influenced human intellect and custom. To the pre-technical mind, it ordered the world and set the rhythm of daily life. For many early peoples, solar observations formed the bases of their agricultural, religious, and ceremonial lives. The power of the Egyptian Pharaoh is underlined by the belief that he was the son of the sun god Ra. Images and designs found world-wide depict the sun. The sun governed conceptions of time; ancient observatories measured the time in terms of seasons, while, at least in antiquity, only the sundial existed to measure the time during a single day. The ancient Greeks considered the sun to be unchangeable and divine, and Plato likened the highest form of human understanding, the understanding of unchanging truths, to the sun. In many cultures, particularly more northern ones, the end of winter is marked by celebrations. Even the Copernican revolution played a role in the conception of the central power of the sun, as the gravity of the sun became the literal controller of the solar system, displacing the changeable and corruptible earth from the centre position. It is little wonder, then, that the absolute monarch Louis XIV assumed the title of Sun King.

Kristen L. Zacharias


Smithsonian Institution (1981). Fire of life. The Smithsonian book of the sun. W. W. Norton and Company, New York and London.

See also ageing; biological rhythms; skin.