How much time is required between repeated exposures to sun to prevent sunburn

How much time is required between repeated exposures to sun to prevent sunburn

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After being in the sun, how much 'rest' is required before you can expose yourself again?

e.g. from Wikipedia:

Sunburn can occur in less than 15 minutes

So if you were in a place where sunburn would occur after 15 minutes and you stayed in the sun for 10 minutes you shouldn't get sunburn.

How long would you then have to stay out of the sun before you could go into the sun again for 10 minutes without getting sunburn?

I'm pretty sure that 1 minute wouldn't be enough, but 24 hours would be fine, is there any logic as to where between those two figures it actually is?

Erythema Treatment And Prevention

Erythema is a skin condition that results in skin redness or rash. Some of the most prevalent types of erythema include UV-induced erythema, erythema nodusum, photosensitivity, and erythema multiforme.

UV-induced erythemais sunburn: when an individual develops a burn from ultraviolet radiation resulting in irritation of the skin, redness and inflammation. The is by far the most common type of erythema.

Erythema nodusum is skin inflammation that tends to center around the fatty layers of the skin and is characterized by painful lumps usually on the front of the legs.

Photosensitivity from medicationis when an individual is taking medication or has some type of infection that creates sensitivity to the sun and UV rays and results in redness or rash.

Erythema multiforme is caused by various medications, illness, or infection and is characterized by lesions or raised red spots on the skin.

There are many other less-common types of erythema as well.

Other types of erythema include: Erythema chronicum migrans, Erythema infectiosum (also known as fifth disease), Erythema toxicum, Erythema ab igne, Erythema induratum, Erythema elevatum diutinum, Palmar erythema, Erythema gyratum repens, and Keratolytic winter erythema.

What is the difference between tanning and sunburn?

Tanning occurs when special cells in your skin called melanocytes produce a brown pigment called melanin when exposed to sunlight. This is a good thing, because the brown pigment can help to protect your skin from some of the sun’s damage. Burning on the other hand is completely unrelated to tanning – in fact, the redness and soreness caused by sunburn is a sign that the top layer or layers of your skin have been damaged beyond repair.

It’s a common myth that if you get sunburn, it will ‘go brown’ later. Some people believe this falsehood so much so that they will actively TRY to burn, in the belief that they will develop a tan. This is very dangerous! Once the skin is burnt, it stays burnt and cannot be ‘unburnt’. The only way you will ‘go brown’ later, is when the damaged cells in the top layer of your skin eventually peel away to reveal new skin underneath.

Some people also mistakenly believe that they cannot burn, not realizing that it is entirely possible to burn and tan at the same time. There is of course some natural variation in different peoples’ tolerance to sunburn, but people who tan easily should still be aware that the brown colour of the tan may mask the redness of the burn. If your skin peels after time spent in the sun, it has been burned!


  • It's now safe to use sunscreen on babies. Choose sunscreen designated for infant skin, and one that won't sting baby's eye.
  • All the protection methods explained above still apply, however now sunscreen use should be incorporated.
  • Apply broad-spectrum sunscreen with a minimum SPF of 30, with UVA/UVB protection particularly to areas left uncovered such as baby's hands. Some children experience allergic reactions to various sunscreen ingredients. Test a product first by applying a small amount to a limited area of skin. Choose a product that is hypoallergenic and fragrance-free.
  • Most importantly, sunscreen must be applied 30 minutes before going outside and reapplied every two hours or after swimming or excessive sweating. Products can no longer claim to be waterproof, only water-resistant, and labels must note a time limit of either 40 or 80 minutes before the sunscreen is ineffective.

Protection from other UV sources

For maximal UV protection, acrylic diffusion shields should be placed over bare fluorescent light tubes/bulbs at home and at work to block the small amount of UV irradiation that can leak from such light sources (UVA greater than UVB). In addition, UV blocking plastic adhesive films can be applied to home and automobile windows. A number of companies offer UV light blocking films or plastic shields that can be applied to home and automobile windows. This is important because whereas window and car glass material may offer some shielding against UVB rays, they are transparent for UVA rays.

Symptoms and Causes

What causes skin cancer?

Ultraviolet (UV) radiation from the sun is the number one cause of skin cancer, but UV light from tanning beds is just as harmful. Exposure to sunlight during the winter months puts you at the same risk as exposure during the summertime.

Cumulative sun exposure causes mainly basal cell and squamous cell skin cancer, while episodes of severe blistering sunburns, usually before age 18, can cause melanoma later in life. Other less common causes are repeated X-ray exposure, scars from burns or disease, and occupational exposure to certain chemicals.

Ultraviolet A (UVA) and Ultraviolet B (UVB) rays also affect the eyes and the skin around the eyes. Sun exposure may lead to cataracts, cancer of the eyelids, and possibly macular degeneration.

Who is at risk for skin cancer?

Although anyone can get skin cancer, the risk is greatest in people who have fair or freckled skin that burns easily, light eyes and blond or red hair. Darker-skinned individuals are also susceptible to all types of skin cancer, although their risk is lower.

In addition to complexion, other risk factors include having a family history or personal history of skin cancer, having an outdoor job, and living in a sunny climate. A history of severe sunburns and an abundance of large and irregularly shaped moles are risk factors unique to melanoma.

What are the signs and symptoms of skin cancer?

The most common warning sign of skin cancer is a change on the skin, typically a new mole, a new skin lesion or a change in an existing mole.

  • Basal cell carcinoma may appear as a small, smooth, pearly, or waxy bump on the face, or neck, or as a flat, pink/red- or brown-colored lesion on the trunk, arms or legs.
  • Squamous cell carcinoma can appear as a firm, red nodule, or as a rough, scaly, flat lesion that may itch, bleed and become crusty. Both basal cell and squamous cell cancers mainly occur on areas of the skin frequently exposed to the sun, but can occur anywhere.
  • Melanoma usually appears as a pigmented patch or bump. It may resemble a normal mole, but usually has a more irregular appearance.

When looking for melanoma, think of the ABCDE rule that tells you the signs to watch for:

  • Asymmetry: The shape of one half doesn't match the other.
  • Border: Edges are ragged or blurred.
  • Color: Uneven shades of brown, black, tan, red, white or blue.
  • Diameter: A significant change in size (greater than 6 mm).
  • Evolution: Changes in the way a mole or lesion looks or feels (itchy, bleeding, etc).

Sun Damage: Protecting Yourself

The immediate danger of too much sun is sunburn. If you looked at sunburned skin under a strong microscope, you would see that the cells and blood vessels have been damaged. With repeated sun damage, the skin starts to look dry, wrinkled, discolored, and leathery. Although the skin appears to be thicker, it actually has been weakened and, as a result, it will bruise more easily.

However, the sun's most serious threat is that it is the major cause of skin cancer, which is now the most common of all cancers. Doctors believe that most skin cancers can be avoided by preventing sun damage.

Does the sun have benefits?

You may have been taught that you need sunlight for your body to make vitamin D, because vitamin D is not found naturally in most foods. But today, many foods are fortified with vitamin D during the manufacturing process. Thus, sun exposure is not as important for the body's vitamin D supply as it used to be. Of course, being outdoors makes most people feel good. And playing tennis is better for your health than watching television. But you can still protect yourself from the sun's damaging effects while enjoying yourself outdoors.

How can I avoid the harmful effects of the sun?

Staying out of the sun is the best way to avoid sun damage, but most of us go outdoors regularly. So when you go outside, take these precautions:

  • Always wear sunscreen. Apply it on your skin every day. Make it a habit, as you do with brushing your teeth.
  • Avoid sun in the middle of the day, from about 10 a.m. to 3 p.m. The ultraviolet rays, which cause sunburn, are strongest during this time.
  • Wear protective clothing. When you do go outdoors, especially for long periods in the middle of the day. Long sleeves and slacks, as well as a wide-brimmed hat, help protect your body against the sun's harmful effects.
  • Wear sunglasses that filter UV light.

What is SPF in a sunscreen?

SPF stands for sun protection factor. The SPF number tells you how well the product will protect you from UVB, the burning rays of the sun. (Most sunscreens also absorb ultraviolet "A" rays, or UVA.) The higher the SPF number, the greater the amount of protection. Everyone should use a sunscreen with an SPF of at least 30. If you have had a skin cancer or precancer, you should use a sunscreen with an even higher SPF. Many of the new sunscreens have SPFs of 45 or higher.

Can I use a sunscreen with a low SPF if I don't burn very often?

If you were only trying to avoid sunburn, the answer would be "yes." But protection from sunburn is not the most important reason for wearing sunscreen. You want to reduce damage from the sun. Your skin can be harmed by constant sun exposure, whether or not you see a burn. Remember, sunburn is an immediate reaction, but damage from the sun occurs over a lifetime. If you have had skin cancer or pre-cancer, you should use an SPF of 30 or higher.

Are all the different types of sunscreens safe for me to use?

Yes. There are 2 types of sunscreens: organic (“chemical”) and inorganic (“physical”). They are both safe and they both protect you from sun damage, just in different ways. The level of protection provided by both types of sunscreens depends on their SPF. Recent studies have looked at the absorption of organic sunscreens into the skin, but no harmful effects have been seen. If absorption into the skin is a concern you have, you can use inorganic sunscreens, which have Titanium dioxide or Zinc oxide as their ingredients. As always, it is a good idea to talk to your doctor if you have any questions or concerns.

Who should use sunscreen?

Anyone who spends time outdoors should use a sunscreen. This includes:

  • Men, women, and children.
  • People who tan easily and those who don't.
  • Fair-skinned and dark-skinned people.
  • People who already have tans and sunbathers, gardeners, and skiers.

Are sunscreens safe for children?

Yes. Not only are sunscreens safe for children over age 6 months, if used regularly in childhood they can prevent skin cancers from developing in later life. Recently, a researcher reported that if sunscreens were used regularly by children through the age of 18, there would be a 72% reduction in the cases of skin cancer later in life.

For children under the age of 6 months, protective clothing and shade should be utilized. If these methods are not available, the American Academy of Pediatrics recommends applying a minimal amount of sunscreen with SPF 15 or greater to areas such as the infant’s face and the back of the hands.

How should sunscreen be applied?

Sunscreens are very effective when used properly. Follow these guidelines to give yourself the most protection:

  • Apply the sunscreen at least 20 to 30 minutes before you go outdoors, whenever you will be exposed for 30 minutes or more.
  • Reapply sunscreen every 2 hours while you are outdoors, even if the product is labeled “all-day.” If you get wet or perspire heavily, reapply sunscreen more frequently.
  • Cover all exposed areas, including your ears, lips, face, and back of your hands.
  • Don't skimp apply a generous layer. Smooth it on rather than rubbing it in. A rule of thumb is that 45 ml (a shot glass) of sunscreen is needed to cover all exposed skin to attain the stated level of protection.
  • Women should apply sunscreens under makeup. If you wait to apply sunscreen until you hit the beach, you may already be perspiring, and moisture makes sunscreens less effective.

Should I skip sunscreen if I have sensitive skin?

Some sunscreens contain ingredients that may irritate the skin. If you know you react to specific ingredients, be sure to check the contents on the label. You can also ask your dermatologist to recommend a sunscreen.

However, the sunscreen may not be causing the reaction. Other products that come into contact with your skin, including perfumes, certain medications, and soaps, may make your skin more sensitive. Think about the products you have been using (especially new products), and stop using these products one by one before you stop using the sunscreen. If you are not sure about the side effects of a medication you are taking, consult with your doctor or local pharmacist.

Last reviewed by a Cleveland Clinic medical professional on 10/10/2019.


  • Lim Henry W. Photoprotection and Sun Protective Agents. In: Wolff K, Goldsmith LA, Katz SI, Gilchrest B, Paller AS, Leffell DJ et al, editors. Fitzpatrick's Dermatology in General Medicine. 7th ed. New York, NY: McGraw Hill Medical 2008:Chap 223.
  • Tung R, Vidimos A.Melanoma. In: Carey WD, ed. Cleveland Clinic: Current Clinical Medicine 2010. 2nd ed. Philadelphia, Pa: Saunders Elsevier 2010:section 3.
  • Tung R, Vidimos A.Nonmelanoma skin cancer. In: Carey WD, ed. Cleveland Clinic: Current Clinical Medicine 2010. 2nd ed. Philadelphia, Pa: Saunders Elsevier 2010:section 3.
  • Matta MK, Zusterzeel R, Pilli NR, et al. Effect of Sunscreen Application Under Maximal Use Conditions on Plasma Concentration of Sunscreen Active Ingredients: A Randomized Clinical Trial. JAMA. Published online May 06, 2019321(21):2082–2091.
  • Mohammed, Y. H., Holmes, A., Haridass, I. N., Sanchez, W. Y., Studier, H., Grice, J. E., … Roberts, M. S. (2019). Support for the Safe Use of Zinc Oxide Nanoparticle Sunscreens: Lack of Skin Penetration or Cellular Toxicity after Repeated Application in Volunteers.Journal of Investigative Dermatology, 139(2), 308–315.
  • American Academy of Pediatrics. Sun Safety: Information for Parents. About Sunburn and Sunscreen. Accessed 11/6/2019.

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Cleveland Clinic is a non-profit academic medical center. Advertising on our site helps support our mission. We do not endorse non-Cleveland Clinic products or services. Policy

Cleveland Clinic is a non-profit academic medical center. Advertising on our site helps support our mission. We do not endorse non-Cleveland Clinic products or services. Policy

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Cleveland Clinic is a non-profit academic medical center. Advertising on our site helps support our mission. We do not endorse non-Cleveland Clinic products or services. Policy

Cleveland Clinic is a non-profit academic medical center. Advertising on our site helps support our mission. We do not endorse non-Cleveland Clinic products or services. Policy

Cleveland Clinic is a non-profit academic medical center. Advertising on our site helps support our mission. We do not endorse non-Cleveland Clinic products or services. Policy

Cleveland Clinic is a non-profit academic medical center. Advertising on our site helps support our mission. We do not endorse non-Cleveland Clinic products or services. Policy

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Reader Interactions


David Tucker says

Excellent … and the same reasoning and logic I have been attempting to convince folks of for the last thirty years or more!
I just wish that people would apply some logical thought before swallowing the mainstream, damaging nonsense perpetrated by the misinformed, the vested interests and the manufacturers of toxic products in general.

Lindsey Burkart-Lima says

So, is there any type of sunscreen that is safe and recommended by Weston A. Price Foundation?

Michael Mitchell says

Lara says

Avasol both has carefully chosen ingredients, is reef safe, and doesn’t have wasteful packaging.

Barry g says

Good work. I was unaware of sunscreens toxicity.

I can relate to the feeling no one is listening.
I complained to my countries (canada) health agency about thermal paper, used in bank receipts, store receipts, tickets, etc. Just touching the paper absorbs the chemicals, and are made worse if you use lotions or handsanitizer, which helps absorb chemicals even faster.
I don’t understand why it hasn’t been banned yet.
I really think these health agencies have been completely compromised/hijacked by big business.

Matt says

I’m curious what products do you recommend? I’ve checked natural sun screens and they still have zinc oxide in it? Is zinc oxide only dangerous in nanoparticels as well?

Monica says

Hello, you can look up for Badger sunscreen or organic coconout oil.

Diana Gold Goslin Gold Goslin says

Badger says it contains Zinc Oxide? I’m confused. Thanks!

Eldwin says

Look up sunscreens used by the ancient Egyptians, amerindians, indians, etc.

the egyptians crafted their own sunscreen from lupine, rice bran, and lavender.

amerindians used sunflower oil, aloe vera, western wallflower oil, etc. wheat germ works too, etc etc.

Heidi says

Is badger a good one to use or no?

Vinny says

Try making your own using some clays like Kaolin and French clay mixed into a balm base ⭐️

Geno says

Marcie says

I grew up in the 70’s when using baby oil and iodine was popular. In the 90’s I had several go arounds with squamous cell carcinoma, mainly face and neck. Since then I’ve been slathering on the sunscreen and staying out of the sun during the hottest daylight hours. I’m surprised I haven’t had more issues because the sunscreens are full of toxic chemicals. I found the EWG website and researched all the ones I had use and am currently using. Badger has titanium dioxide with a hazard score of 2, and zinc oxide with a score of 2, and an overall score of 2. Blue Lizard face (which I have been using for several years does not contain titanium dioxide but does contain zinc oxide with a hazard score of 2 and contains octinoxate with a hazard score of 6. They give it an overall score of 2. Should I discontinue using this product?

I was shocked to see the toxic effects of titanium dioxide, because while I’ve never used sunscreen or makeup, I do take daily supplement of magnesium, which list the non-medical ingredients including titanium dioxide. I’ve taken this for years after reading “Magnificent Magnesium” and using Sally Fallon’s cook book “Traditional Nutrition” as a food bible for years. I tried stopping the magnesium and got constipated.

Elizabeth Plourde says

I have just read the recommendations of sunscreen brands in these comments. They appear to have “non-nano” zinc oxide. If they are the micronized, the damage is the same, if not more to our cells. If it is the parent white paste zinc oxide, it harms cellular membranes so they are not the answer for safe sunscreens.
Cover up and eat a high anti-oxidant diet. People show me how they have not burned, as they are amazed themselves that they have not burned due to their eating an anti-oxidant diet, which also protects us from the massive amount of electromagnetic radiations that surround us.

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AS the sun shone from its northernmost point Sunday, bringing the summer solstice - and the longest day of the year - to the Northern Hemisphere, a mysterious hormone that may influence fertility, mood and many other body functions should have reached its annual low point in the blood of those who spent the day outdoors, according to a new finding that may ultimately revolutionize studies of the effects of light on people.

Thanks to the interplay between sunlight and this hormone, the research suggests, Sunday may have been the happiest day of the year for peoples living north of the equator.

The hormone, melatonin, is released from a tiny gland in the brain, called the pineal gland, at night. In some birds and reptiles, this gland is exposed on the top of the head and acts as a ''third eye,'' sending messages about light levels to the brain. The new studies, by Dr. Alfred Lewy, a research psychiatrist, show that exposure to very bright light, such as daylight, can turn off production of melatonin in people.

The finding contradicts the generally held assumption that, through evolution, human beings escaped altogether from the profound hormonal effects of light seen in lower animals, where the visible part of the spectrum regulates the reproductive cycle and many other daily and seasonal rhythms. Body rhythms in people were believed to respond primarily to psychosocial rather than biochemical cues. In fact, however, the new study suggests that people may have adapted to low-level artificial light while remaining sensitive to the much more intense natural cycle of light and dark.

This discovery accounts for several observations of seasonal rhythms in human beings that are similar to those seen in lower animals. These rhythms are more pronounced in people who live far from the equator, suggesting that people are responding to the natural sunlight-dark cycle and are not influenced by artificial lighting.

In Finland, for example, the conception rate peaks in June and July, when Finns are exposed to about 20 hours of sunlight a day. And in the temperate and polar zones, there are seasonal patterns in depression, mania and suicide attempts.

Dr. Lewy pointed out that in New York City the longest day is twice as long as the shortest (the winter solstice) and that people could be expected to secrete more melatonin during the longer nights of winter.

His and other recent studies raise serious questions about the adequacy of most indoor lighting and especially about energy-saving lights that greatly distort the natural spectrum of the sun. The studies show that to affect hormone production, human beings require much brighter light than other animals - three to four times brighter than the lights normally in homes, offices and factories.

''In the last hundred years, we've cut ourselves off from the light we evolved with,'' said Norman T. Gilroy of the Center for Responsive Design, a nonprofit organization in San Francisco. Typical indoor lighting has less than 10 percent of the intensity of outdoor light under the shade of a tree.

Until very recently, only visual acuity was considered in designing indoor lighting. ''People don't need much light to see,'' Dr. Lewy noted. 'ɻut far more intense light is needed to affect melatonin secretion and other circadian rhythms in human beings.''

Dr. Richard J. Wurtman, professor of endocrinology and metabolism at the Massachusetts Institute of Technology, worked out the daily cycle of melatonin secretion and has been studying the biological effects of light for nearly two decades.

He believes that ''we are all unwitting subjects of a long-term experiment on the effects of artificial lighting on health. Until much more is known, we should design indoor lighting to resemble as closely as possible what the sun provides.''

Meanwhile, Dr. Wurtman suggested that people spend as much time as possible outdoors and open curtains and windows to let in unfiltered daylight. Mr. Gilroy recommended using full-spectrum bulbs and special windows that let in ultraviolet rays.

Incandescent bulbs used in homes provide light primarily from the red part of the spectrum. The cool-white fluorescent lamps of offices, factories and institutions emphasize the yellow-green portion. Neither is a notable source of the invisible ultraviolet or infrared radiation found in natural daylight, which has a more even distribution of the spectral colors. The only commercially available light source that approximates the sun's spectrum is ''Vita-Lite,'' a fluorescent bulb that costs three to five times more than cool-white fluorescents, though not more to operate.

Dr. Wurtman and other scientists investigating responses to light believe that people who spend most of their days indoors illuminated by lights that are a poor substitute for the brightness and spectrum of the sun could suffer untoward effects from inadequate light exposure. In experimental animals, prolonged exposure to ordinary indoor lighting has been linked to reproductive abnormalities and enhanced susceptibility to cancer. Various Effects on People

Preliminary studies in people have suggested such problems as increased fatigue, decreased performance, diminished immunological defenses, reduced physical fitness and possibly impaired fertility associated with living and working under incandescent or cool-white fluorescent lights.

Dr. Philip C. Hughes, director of environmental photobiology at the Duro-Test Corporation in North Bergen, N.J., reports that the Russians now prescribe daily doses of ultraviolet light for workers deprived of sunlight exposure. Mr. Gilroy said that several American unions are demanding more healthful lighting in offices.

Bone loss due to inadequate vitamin D formation in the skin also occurs in people expgsed only to ordinary indoor light. Vitamin D, most of which results from exposure to the sun's ultraviolet rays, is needed to absorb calcium from the diet, and many people in northern latitudes become deficient in vitamin D during the winter. The problem is most serious among those who spend the daylight hours indoors, and foods fortified with vitamin D are not always adequate compensation.

Attempts to use the energy-efficient but spectrally limited sodium vapor lamps indoors have met with a slew of immediate health complaints, including eyestrain, headache and nausea, and such lamps have been removed from several schools. This has prompted Dr. Wurtman to urge that ''novel lighting first be tested for health effects before exposing people.''

Different parts of the spectrum have different effects. While the invisible ultraviolet is needed for vitamin D synthesis and suntanning, the visible part of the spectrum influences body rhythms and hormone levels.

Light exerts its internal biochemical effects through the eye. Light passes through the retina to the optic nerve. Part of the optic nerve goes to the brain's vision center and the other part goes to a section of the hypothalamus, the superchiasmatic nucleus, which is the body's internal clock. From this nucleus, the light-generated nerve message travels through the brain to the spinal cord and out a nerve center, called the superior cervical ganglion, that transmits it to the pineal gland.

Dr. Lewy, a psychiatrist who is now director of the Sleep and Mood Disorders Laboratory at the University of Oregon Health Sciences Center, has shown in a preliminary study that some persons who suffer from periodic mental depressions are unusually sensitive to light, though not nearly as sensitive as other animals. In these patients, a lower level of light is required to suppress melatonin secretion. Even those who have recovered seem to retain their supersensitivity to light.

While at the National Institute of Mental Health, Dr. Lewy and his colleagues successfully treated a victim of manic depression who suffered from winter-long depressions that disappeared each spring. By exposing him for six hours a day - three hours at dawn and three at dusk - to high-intensity light that mimics the spectrum of the sun, the man's depression lifted in four days and he has remained well.

''In a sense we made spring come earlier for him,'' the psychiatrist noted. ''We expect to have to repeat the treatment next fall.''

As a result of Dr. Lewy's research, the once dimly lit patient rooms at the mental health institute are now brightly illuminated with high-intensity, full-spectrum bulbs. In his new lab in Portland, Ore., Dr. Lewy plans to test light treatment in patients who become depressed in winter and recover in spring, as well as patients with other kinds of seasonal illnesses, such as ulcers. Many who suffer from cyclical depressions have disturbed circadian rhythms, possibly indicating abnormal pineal function, he said.

Rather than melatonin excess or lack acting as a direct cause of mental ills, Dr. Lewy believes the hormoneserves as 'ɺ marker for what's going on in the brain, the result of many biochemical processes.'' Recently Dr. Lewy has been studying blind persons and has found that they have different melatonin rhythms than those who can see.

Though in nocturnal animals the pineal gland responds to light as dim as that provided by a quarter moon or a candle, in humans, ordinary room lighting does not shut down melatonin formed experiments with people, interrupting their nights by turning on the lights, saw no effect on melatonin levels and assumed that, as intelligent beings, humans were immune to control by so basic an environmental factor as light. A Different Tactic

Dr. Lewy tried a different tactic. After spending three years working with Dr. Sanford P. Markey to perfect a technique for precisely measuring melatonin in blood and urine, he studied volunteers who slept not at night but during the day for a week, until their usual melatonin cycle - high production at night, none during the day -was reversed. This is similar to the effect of traveling from the East Coast to the West Coast. Then he awoke the subjects in the middle of their sleep and exposed them to sunlight. A precipitous drop in melatonin levels occurred. Later, he repeated the studies during the night, using high intensity artificial light. Again, melatonin production shut down when the bright lights went on. These findings were published in the December 12, 1980 issue of Science.

Dr. Harry Lynch, an M.I.T. light researcher who works with Dr. Wurtman, said Dr. Lewy's finding should lead to a wide variety of studies examining how human physiological rhythms respond to light. He explained, ''We may be able to deliberately manipulate pineal function to achieve some sort of therapeutic good, for example, for sleep problems, deities.'' Light manipulation has been used by farmers for years to increase egg laying and milk production, though without understanding how it works.

Dr. Lewy warned, however, that people should not attempt selftreatment by deliberately staring at high-intensity lights or overexposing themselves to sunlight or ultraviolet lamps lest they risk retinal burns, sunburn and damage that could lead to skin cancer.

Associations between sun exposure and other lifestyle variables in Swedish women

Sun exposure is associated with risk of several chronic diseases including cancer. The study aim is to investigate whether sun behaviors are related to other lifestyle risk factors of cancer.


We analyzed data collected in 2003–2004 by self-completed questionnaire from 34,402 Swedish women aged 40–61 years, who comprised 70% of a cohort of originally recruited from a population registry in 1991–1992 (n = 49,259). Participants were asked about annual number of sunburns and annual number of weeks of swimming and sunbathing during 1991–2002, solarium use during 1991–1998 and current sunscreen use.


Compared to non-drinkers, the prevalence ratio (95% CI) in women who drank >10 g of alcohol per day was 1.64 (1.49, 1.81) for having >1 sunburn per year, 1.39 (1.29, 1.51) for swimming and sunbathing >2.5 weeks per year and 1.55 (1.41, 1.70) for using a solarium >1 time per 2 months, adjusting for demographic and lifestyle variables. Tobacco smokers were less likely to report sunburn and to use sunscreen, and more likely to sunbath and use solaria, compared with non-smokers. Physical activity was associated positively with swimming and sunbathing, and with the separate use of solaria and sunscreens, but not with number of sunburns. The lifestyle variables that explained most of the variation in sun behavior were alcohol and smoking.


Our results suggest that alcohol consumption and tobacco smoking are potential lifestyle confounders which should be adjusted in studies investigating the association that sun and/or solarium exposure may have with risk of several cancer sites.