June 2016

Is sunlight more helpful than harmful for our mental and physical health?

 

Introduction

For many years people have been warned about the dangerous effects of sunlight, such as premature skin aging, eye damage and skin cancer. They have been encouraged to stay away from direct sunlight, block out the sun with sunglasses, sun protection and clothes. In spite of this, researches have been stacking up, that show the importance of the sunlight for our physical and mental health, with regard to depression, cognitive function and arthritis, among other things.

In what way can the sunlight be harmful for our health and in what way can it be helpful for our health?  Is it more harmful than helpful? On the next pages these questions will be answered, but first, the energy and radiation of the sun will be discussed.

 

The sun and its energy

The sun´s energy drives almost all of the earth´s systems, such as the climate systems, ecosystems and hydrologic systems, and life on earth would not exist without the sun.19 Solar energy is created at the core of the sun when hydrogen atoms are fused into helium by nuclear fusion.(69)

The energy that radiates from the sun, commonly called sunlight, is a mixture of electromagnetic waves ranging from infrared (IR) to ultraviolet rays (UV). It includes visible light, which is between IR and UV on the electromagnetic spectrum. It also emits cosmic rays, gamma rays and x-rays. All of the cosmic-, gamma- and x-rays is reflected or absorbed when it strikes the atmosphere that surrounds the earth.(16 17) Some of the infrared rays, ultraviolet rays and visible light reach the earth. There are different wavelengths of UV rays. UVA (321-400 nm), UVB (290-320) and UVC (200-280 nm). All UVC is absorbed by the ozone layer and  approximately 0,1% of UVB and 5% of UVA reach the earth surface.(16 41)

The various solar radiation waves affect our body in different ways depending on the wavelength. Some, for example UVB rays, only enter the epidermis, but others, like UVA rays, permeate the dermis, and both the visible light and infra-red light penetrate deep into the body.(16)

This broad spectrum of light that radiates from the sun influences people’s health in various ways. Some effects can be harmful to our health as in the case of skin cancer, but others are highly beneficial, such as the production of Vitamin D. This will be addressed next.

 

Harmful effects of the sunlight

Prolonged exposure to sunlight can cause sunburn, skin cancer, weakening of the immune system, eye damage and premature skin aging, according to the World Health Organization (WHO).(1) WHO advises to avoid direct sunlight between 10am and 4pm, when the UV rays are strongest, but if people do venture out during these hours, they should adopt sun safety practices if the UV level is moderate or stronger.  WHO explains sun safety practices as using sun protection SPF 15+, using sunglasses with 99-100% UVA and UVB protection, seeking shade when the sunlight is the most intense, wearing protective clothing and a hat with a wide brim. People are also advised to avoid sunlamps and tanning parlors.(1) A definition of UV levels can be found on the website of EPA, the Environmental Protection Agency of the USA.2 The levels are from one to eleven and moderate levels of UV are considered to be levels three to five. The UV rays levels are highest at the equator regions, because there the sun is directly overhead which means the UV rays have a shorter distance to travel to the ground. UV levels are also elevated at high altitudes because thinner atmosphere provides less protection.(57)

Skin cancer has been on the rise since the 1970´s3 and has increased by 44% from 2002 to 2011, according to a study published in the American Journal of Preventive Medicines, while all other cancers increased by 32% over the same period of time.(4)

The cause for this increase in skin cancer is thought to be twofold: changes in behavior, such as more sunbathing and particularly the use of sun beds5; and a hole in the ozone layer over the South Pole,(63) which exposes us to increased UV rays.(6)  It is thought that both UVA and UVB rays, can contribute to skin cancer. UVA contribution to skin cancer is indirect by penetrating deep into the dermis and generating DNA-damaging molecules such as hydroxyl and oxygen radicals. Too much UVB radiation can cause sunburn, which can lead to direct DNA damage and cause various skin cancers.(13) Other factors that may increase the risk of skin cancer include previous skin cancers, previous radiotherapy treatment, lowered immunity, overexposure to chemicals at work and rare genetic conditions.(7)

A study from 2009 shows that maybe the sunlight is not to blame for the increase in melanoma skin cancer. The increase in melanoma incidents was shown to be only in the stage 1 of the disease, which is actually not cancer, but benign lesions.(37) The study also shows that these lesions are hardly caused by sun exposure as they were found on places on the body that are rarely exposed to sunlight. So it seems that the diagnosis of melanoma skin cancer is sometimes incorrect and that benign skin lesions are diagnosed as melanoma.(37) Another fact that casts doubt on the sunlight being the root cause for melanoma is that the increase is much more common amongst people who work inside than those who work outside.(38) The indoor workers get three to nine times less solar radiation, yet only indoor workers have an increasing incidence of melanoma. The reason may be, that as only UVA rays go through glass, the lack of UVB rays and the consequent lack of Vitamin D production cause these increased incidents of melanoma skin cancer. UVA rays can pass through windows, penetrate deep into the skin, cause mutations and can break down vitamin D in the skin and capillaries. Vitamin D can help fight the melanoma cancer cells and is created by outdoor UVB sunrays.(38)

Sunlight can also harm our eyes if we don´t practice safe behavior in our outdoor activities, and eye damage like cataract has been increasing the last three decades.(8) Too much exposure to UV light increases danger of eye diseases like cataract and growth on the eye. Surfers, skiers, fishermen, farmers, or anyone who spends long hours under the mid-day sun or in the UV-intense conditions found near rivers, oceans, and mountains, must be very careful and protect their eyes with UV protecting sun glasses and broad-brimmed hats. Thin clouds don´t protect the eyes from the UV rays.(9) But it is not only the sunlight that can cause cataract and other eye diseases. Other factors are for example medication like corticosteroids, smoking, alcohol, diabetes, and radiation treatment for some types of cancers.(10 42)

The aging effect of sunrays on the skin has long been known. Aging of skin can be divided into four different categories: wrinkles, lack of firmness of cutaneous tissues (ptosis), vascular disorders, and pigmentation heterogeneities.(11) Factors that have important effects on skin aging are for instance tobacco, diet, stress, and sun exposure. Of these, the sun exposure is responsible for most of the visible aging signs on skin, according to a study from 2013.(11)

The negative effects of sunlight on the immune system are alterations of the activity and distribution of some of the cells responsible for triggering immune responses in humans. The results can be outbreaks of cold sores, as in the case of Herpes Simplex infections. Applying sunscreen seems to prevent this kind of outbreak of cold sores.12   

As above-mentioned examples show, there is no doubt that excessive sun exposure can cause various harm to people’s health. However, excessive UVR exposure accounts for only 0.1% (1,5 million DALY´s) of the total global burden of disease in disability-adjusted life years (DALYs), according to a WHO report from 2006, The Global Burden of Disease Due to Ultraviolet Radiation. In contrast, the same WHO report noted that a larger annual disease burden of 3304 million DALYs worldwide might result from very low levels of UVR exposure.(71) This burden subsumes major disorders of the musculoskeletal system and possibly an increased risk of various autoimmune diseases and life-threatening cancers. In contrast, the diseases caused by too much exposure to UVR are not categorized as major diseases, apart from malignant melanoma.(13)

And now, looking at in what way the sunlight can be helpful for people’s health will explain why the global burden of disease can increase if sun exposure is lacking.

 

Helpful effects of sunlight

Are there more helpful than harmful effects on health of sun exposure? Many studies demonstrate how sunlight exposure can be beneficial both for mental and physical health as will be demonstrated in this chapter. Too little exposure to sunlight has been linked to diseases and disorders like Multiple Sclerosis (MS), depression, ADHD(60), insomnia, auto-immune diseases, infectious diseases, cardiovascular diseases and cancers like colon cancer and breast cancer.(59)

 

Vitamin D

One of the best-known benefits of the sunlight is the production of Vitamin D in the skin, through photosynthetic reaction, triggered by UVB radiation from the sun, where cholesterol absorbs the UVB photons and transforms them to Vitamin D3.(14 63)

There are two forms of vitamin D: ergocalciferol (vitamin D2) and cholecalciferol (vitamin D3), of which the latter is considered better at raising levels of Vitamin D in the blood.(35)

Vitamin D is important for many biological processes in the body and the body needs it to fight off infection.(14) At least 1,000 different genes governing virtually every tissue in the body are now thought to be regulated by D3, the active form of the vitamin, including several involved in calcium metabolism and neuromuscular and immune system functioning.(13)

Vitamin D deficiency is a worldwide health problem as it can lead to serious and chronic diseases, including autoimmune diseases, infectious diseases, cardiovascular disease, and deadly cancers like colorectal cancer.(15 16) And although sunlight is the best source of Vitamin D, it is surprising that living close to the equator is no guarantee for high levels of this sunlight vitamin, as a study in 2016 on Vitamin D levels in the Cambodian population, reveals.(43) The study shows Vitamin D deficiency in Cambodian women which is probably due to lifestyle choices such as avoiding the sunlight and working indoors.(43) Most often, Vitamin D deficiency is due to lack of outdoor sun-exposure. The efficiency of production depends on the number of UVB photons that penetrate the skin, a process that can be affected by clothing, excess body fat, sunscreen, the skin pigment melanin, the time of year, the time of day, and the latitude.(15)

People who do not receive enough vitamin D from sun exposure need at least 1000 IU/d of Vitamin D. By increasing the intake of this vitamin, it is estimated that there is a 30 to 50% reduction in risk of developing colorectal, breast, and prostate cancer.(15)

Vitamin D deficiency is associated to arterial dysfunction(34) and moreover, it seems that the risk for developing both multiple sclerosis (MS) and type 1 diabetes increases the further people live from the equator.16 In a study from 2006, the conclusion is that lack of Vitamin D in nutrition and low serum Vitamin D could be associated with arteriosclerosis and endothelial dysfunction.(34)

Vitamin D seems to play an important role in auto-immune diseases. The link is striking in MS and diabetes 1, but not as obvious in other autoimmune disease, like Inflammatory Bowel Disease, Rheumatoid Arthritis and Crohn’s Disease, although there is some association.(46)

In a research from 2008, Michael F. Holick concludes that people who do not receive adequate vitamin D from sun exposure need at least 1000 IU/d of Vitamin D. By increasing the intake of this vitamin, it is estimated that there is a 30% to 50% reduction in risk for developing colorectal, breast, and prostate cancer.(15) The optimal doses of Vitamin D is disputed. In the Journal of the American College of Nutrition, published in 2014 it is stated that to meet physiological needs and to support optimal health, we need approximately 6000IU of Vitamin D, per day from all sources, food, sun and supplements.31 According to a study in 2015, on 40 people with MS, Vitamin D given in the doses of 10,400 IU a day, can decrease some of the specific immune system cells associated with MS.(32)

Still, the official recommendation in Britain for daily doses of Vitamin D is only 400 IU for people older than 65 and lactating women and adults are recommended not to take more than 1000 IU per day.(33)

The major source of vitamin D for most children and adults is sunlight as relatively few foods contain natural Vitamin D. The best food sources include wild caught salmon, other oily fish, cod liver oil and sun-dried mushrooms.(16) As a result, some countries like Iceland and UK encourage the fortification of milk with Vitamin D.

Effects on mood and cognitive function

The effect of the weather on our mood has long been of interest, particularly in countries such as the Nordic countries, where the weather can be unpredictable and direct sunlight is scarce. In the following chapter a number of studies reveal that there really is some connection between sunlight, mood and cognitive function.

Mood

Our eyes are important in our mental health because light and darkness act on the retina and trigger the release of hormones and neurotransmitters, which help us stay awake, feel good, be focused or fall asleep.(20)

Our retinas are covered with a light sensitive network of nerve cells called photosensitive retinal ganglion cells (pRGCs). They react to bright light, particularly blue light, and influence our 24h circadian rythm.(53) When people are exposed to sunlight or very bright artificial light in the morning, their nocturnal melatonin production occurs sooner, and they fall asleep more easily at night.(13) Bright light during the day, can lead to better brain performance, decrease day-time sleepiness and lead to better sleep.(53) The pRGCs sensitivity to light decreases with age and therefore we need more light, as we grow older for optimal health. For example, 85 years old need 530-1340 lux,(51) which is similar to the illumination in drawing offices and bright supermarkets.52 Residential illumination is much lower, typically averaging only 100 lux.(51)

When sunlight illuminates the pRGCs, they stimulate the production of various neurotransmitters and hormones like cortisol and serotonin, which are important for our physical and mental health.(21 54) The release of serotonin in the brain (20) helps us feel calm and focused, but insufficient levels of serotonin can result in depression, lack of energy, sleep problems, mood swings and poor impulse control.(21) Unbalanced secretion of cortisol can for example cause behavioral problems (22) and weight problems.(64)

At night the dimmer light helps us feel sleepy by triggering the release of the hormone melatonin in the brain(.55) The shift from serotonin secretion to melatonin secretion at night time forms part of the circadian rhythm and influences our sleep quality.(65) Melatonin can also suppress cancer cell growth and even cause cancer cell death and has various other important functions like absorbing free radicals.(55)

If a person does not get enough sunlight exposure, the result can be insufficient levels of serotonin, which can cause seasonal affective disorder (SAD), a kind of depression.20 Therefore, people with SAD have regular bouts of depression occurring during winter and remissions in spring or summer.28 Studies have revealed that people with SAD have higher levels of Serotonin transporter protein in winter than in summer. Serotonin transporter protein takes the serotonin back into the nerve cells, where it is not active. The sunlight keeps the levels of Serotonin transporter protein low.(24 40)

Bright light can therefore work as an antidepressant by eliciting anti-depressant effect in the brain.

Still, the weather has not the same impact on everyone. According to a study from 2008, the daily influence of the weather has stronger impact on a person’s negative mood, rather than helping one’s positive mood. Higher temperatures raise a person’s mood up when it’s low, while things like wind or not enough sun made a low person feel even lower.(66)

 

Cognitive Function

Several studies have been carried out the last years on the effect of climate on mood and cognitive function. (28) These studies show that pleasant weather, more time spent outdoor or more sun exposure is associated with better mood, better cognitive function and better memory. A study from 2005 (25) suggests that pleasant weather improves mood and broadens cognition. It is also concluded that clearer days and warmer temperature, with more time spent outside is related to more openness to new information and more flexible thinking styles.(25)

Sound sleep is very important for good brain function and has been shown to influence the brain´s plasticity and memory consolidation.(27) For sound sleep we need sunlight because the sunlight affects the circadian rhythm, according to a study from 2012, where it is concluded that lower levels of solar radiation are related to increased odds of incidents of cognitive impairment.(30) Possible pathways for this relationship between sunlight and cognitive decline are thought to be either through vitamin D metabolism or circadian rhythm regulation via the suprachiasmatic nucleus or SCN, which is in the part of the brain called hypothalamus. The nuclei SCN is known to be involved in the relationship between light and depression and is affected when certain wavelengths of light enter our eyes.(30) Sometimes the SCN is called the “pacemaker” in the brain.(53)

 

Other helpful effects of the sunlight

Different wavelengths of the sunlight affect a variety of biological processes including increasing the expression of proopiomelanocortin (POMC) gene.(16) POMC triggers signaling pathways that control as disparate functions as the release of cortisol, which, along with other things, helps maintain blood sugar levels; the release and production of the pigment melanin; and stimulating signaling for pain relief through opioid receptors in the brain.(18)

The POMC gene also affects vitamin D production, nitric oxide (NO) release and production, carbon monoxide production and enhances wound healing.(16)

Still another beneficial role of the sunlight is the UVA´s induction of NO production.(50) NO reacts with a number of targets, such as haem groups, cysteine residues and iron and zinc clusters. This wide range of targets for NO might explain the multiple health benefits it promotes, such as vasodilatation, immune defense and cell motility.(50) A study from 2014 demonstrates that irradiation with UVA, equivalent to natural sunlight exposure for 30 minutes at noon on a sunny day in Southern Europe, causes vasodilation in the arteries and reduces blood pressure. The UVA rays induce the release of nitric oxides from stores in the skin. This means that adequate sun exposure on skin may have a role in cardiovascular homeostasis.(61) This is confirmed in a study published 2016, where it is stated that sunlight has health benefits, independent of vitamin D and thus cannot be reproduced by oral supplementation.(70) Epidemiological data show a correlation between increased sun exposure and reduced cardiovascular mortality.(70)

The sunlight furthermore, has disinfectant effects and in Africa, Asia and South-America, water disinfectant technique, using the sunlight has become very popular among poor people.(68)

 

Zones with little direct sunlight

As previous chapters have established, the skin needs direct sunlight to form D3, and people living in areas in latitude higher than 37°, do not get adequate sunlight during the winter months for Vitamin D production.(13 67) To put these information into perspective, this means that the greater part of Europeans do not get enough Vitamin D during winter as the latitude 37°N crosses the southern cities of Athens in Greece, Seville in Spain and Sicily in Italy.(47)

Vitamin D deficiency and living at higher latitudes where direct sunlight is scarce, is associated with increased risk for many chronic diseases including autoimmune diseases, some cancers, cardiovascular disease, infectious disease, schizophrenia and type 2 diabetes.(45 56)

A study from 2016, on the prevalence of MS in people living in Wales, shows that the incident rate is directly connected with latitude and sunshine hours. The number of incidents increases as one moves north within Wales.(44) Chances of getting MS also vary greatly by ethnicity. White people are most at risk, while people from sub-Saharan Africa and south Asia suffer least. But a study on prevalence rates in east London found that the risk for Asian and black people born in London to get MS was several times the risk of those still living in their countries of origin. The study concludes that lifestyle and environmental factors like little direct sunlight are factors in the prevalence of MS, as well as ethnicity.(39)

In the Nordic countries, lack of sunlight is prevalent during the winter months and although the sun does shine, it is so low on the horizon, that it often does not reach the ground, as mountains and buildings block the sunlight. An architect working with light and shadows  recommends that the bedroom face east for getting sunlight through the window in the morning. The same goes for the kitchen, where people tend to spend the most time in the morning.(22) The reason for this designing is the brain´s need for “light shock” in the morning to stop the melatonin production to help us face the day alert and well awake. (23) In the Northern Countries the skies are very often cloudy, which means even less direct sunlight, so “light shock” in the morning is hardly an option in the winter months.

Population studies have shown consistently in many Northern countries that the majority of the population is vitamin D deficient. Therefore it is proposed to restore levels of Vitamin D, to >100 nmol/L for optimum function of the immune system. 2000 IU of vitamin D3 achieve these levels in about 80% of patients and could decrease incidents of auto-immune diseases.(46)

Many studies and meta-analysis show tendency for increased incidents of schizophrenia in higher latitudes and colder climates and the suspected culprit is lack of Vitamin D.(48 56) Someone born during the winter months, has a higher risk of schizophrenia than someone born in summer, and dark-skinned people living in northern countries are three to four times more likely to develop schizophrenia than light-skinned natives.(58)

A study from 2005 (49), presents some evidence of association between low UVR exposure and/or low levels of Vitamin D and the onset of three auto-immune mediated diseases, Diabetes 1, Multiple Sclerosis and Rheumatoid Arthritis. This study, also proposes that public sun exposure recommendations need to take into account the adverse effects of insufficient UVR on health.(49) Evidence is increasingly pointing towards a significant role of vitamin D in reducing the incidence and burden of autoimmune diseases.(50)

Finally, there are more incidents of ADHD in areas with low sun intensity, which might be explained by the beneficial effects of the sunlight on the circadian clock, which is associated with ADHD.(60)

 

Conclusion

The harmful effects of the sun are well known. Too much sun exposure can cause skin cancer, negative immune responses, aging of the skin and eye diseases like cataract.  The reason for the negative effect of sunlight may be inappropriate sun exposure such as staying outside in strong sunlight for hours when the UV levels are moderate or stronger, without protection. But maybe the sun has wrongly been blamed for some of these health problems as in the case of the increase in melanoma skin cancer. According to the World Health Organization, the global burden of disease, due to ultraviolet radiation is larger because of too little sun exposure, than because of too much sun exposure. This is because too little sun exposure can lead to more serious diseases like autoimmune diseases than too much sun exposure.

The helpful effects of the sun on people´s health are variegated. The direct sunlight is needed for the skin´s formation of Vitamin D, which has multiple beneficial effects on health. The bright sunlight triggers the production of hormones and neurotransmitters that affect mood and sleep, and scarce sunlight has been associated with ADHD, schizophrenia and a decline in cognitive function. Sunlight has also been shown to have helpful effects on autoimmune diseases like MS and diabetes as well as some cancers like breast cancer.  Finally the sunlight has disinfectant effects and can be helpful to disinfect drinking water.

From all the above-mentioned factors, the conclusion is that the sun is more helpful than harmful for people´s physical and mental health but they need to be sensible in their sun exposure and avoid sunburn and prolonged sun exposure when the UV level is moderate or stronger.  

 

 

References

1. World Health Organization. (2016) “Ultraviolet radiation and the INTERSUN Programme.” http://www.who.int/uv/health/en/

2. EPA. (January 5, 2016) “UV Index Scale.” https://www.epa.gov/sunsafety/uv-index-scale-1

3. World Health Organization. (2002). “Helping people reduce their risks of skin cancer and cataract.”

 http://www.who.int/mediacentre/news/releases/who60/en/

4. Skin Cancer Foundation Journal. (2015) “Skin Cancer World News Summary”, p.8. http://skincancer.epubxp.com/i/526209-may-2015/8

5. Medical News Today. (April 3, 2012). “Alarming Rise in Skin Cancer Rates.” http://www.medicalnewstoday.com/articles/243652.php

6. World Health Organisation. (2016). “Climate change and human health - risks and responses. Summary.” http://www.who.int/globalchange/summary/en/index7.html

7. McMillan Cancer Support. (February 28, 2014). “Causes And Risk Factors of Skin Cancer.” https://www.macmillan.org.uk/information-and-support/skin-cancer/diagnosing/causes-and-risk-factors/potential-causes-of-cancer/causes-risk-factors-skin-cancer.html 

8. Collogly HE, Hodge DO, St Sauver JL, Erie JC.  (2013). “Increasing incidence of cataract surgery: population-based study.” Journal of cataract and refractive surgery. 2013 Sep;39(9):1383-9 http://www.ncbi.nlm.nih.gov/pubmed/23820302

9. Turbert, David. (August 28, 2014). “The Sun UV Radiation and Your Eyes.” American Academy of Ophthalmology. http://www.aao.org/eye-health/tips-prevention/sun

10. The New York Times. (2008). “Cataracts In-Depth Report.” http://www.nytimes.com/health/guides/disease/cataract/print.html

11. Frederic Flament, Roland Bazin, Sabine Laquieze, et.al. (2013). “Effect of the sun on visible clinical signs of aging in Caucasian skin.” Clinical, Cosmetic and Investigational Dermatology. Volume 6, 2013, page 221-232. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3790843/

12. World Health Organisation. (2016). “The Known Health Effects of UV.” http://www.who.int/uv/faq/uvhealtfac/en/index4.html

13. Environmental Health Perspectives. (2008). “Benefits of sunlight: a bright spot for human health.” April 2008, Vol. 116 Issue 4, pA160-A167. Page 2-3.

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2290997/

14. Vitamin D Council. “What is Vitamin D?” http://www.vitamindcouncil.org/about-vitamin-d/what-is-vitamin-d/

15. Holick, Michael F. (2008). “Vitamin D and Sunlight: Strategies for Cancer Prevention and Other Health Benefits.” CJASN. September 2008. Vol. 3. No 5. 1548-1554. http://cjasn.asnjournals.org/content/3/5/1548.short

16. Holick, Michael F. (2016). “Biological Effects of Sunlight, Ultraviolet Radiation, Visible Light, Infrared Radiation and Vitamin D for Health.” Anticancer Research. March 2016 vol. 36 no. 3 1345-1356. http://ar.iiarjournals.org/content/36/3/1345.long

17. Villanueva, John Carl. (Mars 19, 2010). “Radiation From the Sun.” Universe Today.  http://www.universetoday.com/60065/radiation-from-the-sun/

18. Genetics Home Reference. (April 26, 2016). “POMC proopiomelanocortin”. https://ghr.nlm.nih.gov/gene/POMC

19. Pidwirny, Michael. (May 24, 2012). “Solar Radiation.” The Encyclopedia of Earth. http://www.eoearth.org/view/article/156098/

20. Nall, Rachel. (November 9, 2015.) “Sunlight and Serotonin.” Healthline. http://www.healthline.com/health/depression/benefits-sunlight#Overview1

21. Haugan, Idu. (Januar 18, 2013). “Beating the Winter Blues.” ScienceNordic. http://sciencenordic.com/beating-winter-blues

22. Ruttle, Paula L. et al. (2011) “Disentangling psychobiological mechanisms underlying internalizing and externalizing behaviors in youth: Longitudinal and concurrent associations with cortisol.” Hormones and Behavior. Volume 59, Issue 1, January 2011, Pages 123–132 http://www.sciencedirect.com/science/article/pii/S0018506X10002795

23. Haugan, Idun. (January 11, 2013). “Mörketidsmedisin.” Forskning.no. http://forskning.no/forebyggende-helse-medisinske-metoder/2013/01/morketidsmedisin

24. Whiteman, Honor. (October 25, 2014). “Seasonal depression 'caused by increased levels of serotonin transporter protein.” MNT. http://www.medicalnewstoday.com/articles/284195.php

25. Keller, Matthew C. et.al. (2005). “A Warm Heart and a Clear Head. The Contingent Effects of Weather on Mood and Cognition.” Psychological Science. September 2005 vol. 16 no. 9 724-731. http://pss.sagepub.com/content/16/9/724.long

26. Allen, Margaret A. and Gloria J. Fischer.(1976). “Ambient Temperature Effects on Paired Associate Learning.” Ergonomics. Volume 21, Issue 2, 1978. http://www.tandfonline.com/doi/abs/10.1080/00140137808931700

27. Walker, Matthew P. and Stickgold, Robert. (2004). “Sleep-Dependent Learning and Memory Consolidation.” Neuron. Volume 44, Issue 1, 30 September 2004, Pages 121–133 http://www.sciencedirect.com/science/article/pii/S0896627304005409

28. Kent, Shia T; McClure, Leslie A. et.al. (2009). “Effect of sunlight exposure on cognitive function among depressed and non-depressed participants: a REGARDS cross-sectional study.” Environmental Health. July 28, 2009. http://ehjournal.biomedcentral.com/articles/10.1186/1476-069X-8-34

29. Intro To Astronomy. (2002). “The Suns Structure and Nuclear Fusion.” http://earthguide.ucsd.edu/virtualmuseum/ita/07_2.shtml

30. Kent, Shia T, Kabagambe, Edmond K. et.al. (2012). “The relationship between long-term sunlight radiation and cognitive decline in the REGARDS cohort study.” International Journal of Biometeorology. http://link.springer.com/article/10.1007/s00484-013-0631-5

31. Baggerly Carole A., Cuomo Raphael E. et al. (2015) “Sunlight and

Vitamin D: Necessary for Public Health”, Journal of the American College of Nutrition, 34:4,

359-365, DOI: 10.1080/07315724.2015.1039866

http://www.tandfonline.com/doi/pdf/10.1080/07315724.2015.1039866

32. Bhargava Pavan, Sotirchos Elias et al. (2015.) “High-dose vitamin D supplementation reduces IL-17-producing CD4+ T-cells and effector-memory CD4+ T-cells in multiple sclerosis patients.” Neurology. April 6, 2015 vol. 84 no. 14 Supplement S38.001

http://www.neurology.org/content/84/14_Supplement/S38.001.short

33. NHS. (June 18, 2013). “News analysis: Health claims about vitamin D examined.” http://www.nhs.uk/news/2013/06June/Pages/health-claims-about-vitamin-D-examined.aspx

34. London ,Gérard M., Guérin, Alain P. et al. (February 2007). “Mineral Metabolism and Arterial Functions in End-Stage Renal Disease: Potential Role of 25-Hydroxyvitamin D Deficiency.” JASN. February 2007 vol. 18 no. 2 613-620. http://jasn.asnjournals.org/content/18/2/613.abstract?ijkey=72250c839ff3b7bea5494526585a58cf3d6e3091&keytype2=tf_ipsecsha

35. University of Maryland Medical Center. (April 2, 2016). “Vitamin D.” http://umm.edu/health/medical/altmed/supplement/vitamin-d

36. Krzyścin,, Janusz W., Guzikowski, Jakub and Rajewska-Więch, Bonawentura. (2016). “Optimal vitamin D3 daily intake of 2000 IU inferred from modeled solar exposure of ancestral humans in Northern Tanzania.” Journal of Photochemistry and Photobiology. Volume 159, June 2016, Pages 101–105.  http://www.sciencedirect.com/science/article/pii/S1011134415301561

37. Levell NJ, Beattie CC, Shuster S, Greenberg DC. (2009). “Melanoma epidemic: a midsummer night's dream?” The British Journal of Dermatology. 2009 Sep;161(3):720. http://www.ncbi.nlm.nih.gov/pubmed/19519827

38. Dianne E., Landry, Robert J., Lucas, Anne D. (2009). “Increased UVA exposures and decreased cutaneous Vitamin D3 levels may be responsible for the increasing incidence of melanoma.” Medical Hypotheses. April 2009, Volume 72, Issue 4, Pages 434–443.  http://www.medical-hypotheses.com/article/S0306-9877%2808%2900599-9/abstract

39. Albor C, du Sautoy T, Kali Vanan N. et al. (2016). “Ethnicity and prevalence of multiple sclerosis in east London.” Multiple Sclerosis Journal. March 17, 2016. http://msj.sagepub.com/content/early/2016/03/16/1352458516638746.long

40. Mc Mahon B., Andersen SB., Madsen MK, Hjordt LV et al. (2016). “Seasonal difference in brain serotonin transporter binding predicts symptom severity in patients with seasonal affective disorder.” Brain. A Journal on Neurology. 2016 May;139 (Pt 5):1605-14. doi: 10.1093/brain/aww043 http://www.ncbi.nlm.nih.gov/pubmed/26994750

41. WHO. (2016) “UV radiation.” http://www.who.int/uv/faq/whatisuv/en/index2.html

42. National Eye Institute. (September 2009). “Facts About Cataract.” https://nei.nih.gov/health/cataract/cataract_facts70

43. Smith G, Wimalawansa SJ, Laillou A. et al. (2016). “High Prevalence of Vitamin D Deficiency in Cambodian Women: A Common Deficiency in a Sunny Country.” Nutrients. 2016May 12;8(5).290 http://www.mdpi.com/2072-6643/8/5/290

44. Balbuena LD, Middleton RM, Tuite-Dalton K. et al. (2016). “Sunshine, Sea, and Season of Birth: MS Incidence in Wales.” PloS One. 2016 May 16;11(5). http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0155181

45. Wacker, Matthias and Holick, Michael F. (2013) “Sunlight and Vitamin D. A global perspective for health.” Dermatoendocrinology. 2013 Jan 1; 5(1): 51–108. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3897598/

46. Schwalfenberg, G. K. (2012). Solar Radiation and Vitamin D: Mitigating Environmental Factors in Autoimmune Disease. Journal of Environmental and Public Health, 2012, 619381. http://doi.org/10.1155/2012/619381

47. Maps of World. “Latitude and Longitude Finder.” http://www.mapsofworld.com/lat_long/

48. Kinney Dennis K., Teixeira Pamela, Hsu Diane, Napoleon Siena C. et al. (2009) “Relation of Schizophrenia Prevalence to Latitude, Climate, Fish Consumption, Infant Mortality, and Skin Color: A Role for Prenatal Vitamin D Deficiency and Infections?” Schizophrenia Bulletin. 2009, 35 (3): 582-595.doi: 10.1093/schbul/sbp023 http://schizophreniabulletin.oxfordjournals.org/content/35/3/582.long

49. Ponsonby AL, Lucas RM, van der Mei IA. (2005) “UVR, vitamin D and three autoimmune diseases--multiple sclerosis, type 1 diabetes, rheumatoid arthritis.” Photochemistry and Photobiology. 2005 Nov-Dec;81(6):1267-75. http://onlinelibrary.wiley.com/doi/10.1562/2005-02-15-IR-441/full

50. Juzenien Asta e. and Moan Johan. (2012) “Beneficial effects of UV radiation other than via vitamin D production.” Dermatoendocrinol. 2012 Apr 1; 4(2): 109–117. doi:  10.4161/derm.20013 http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3427189/

51. P L Turner, M A Mainster (2008) “Circadian photoreception: ageing and the eye’s important role in systemic health.” British Journal of Ophthalmology. 2008;92:1439-1444 doi:10.1136/bjo.2008.141747

52. Autodesk Education Community. (2015) “Measuring Light Levels” http://sustainabilityworkshop.autodesk.com/buildings/measuring-light-levels

53. Dijk Derk-Jan and Archer Simon N. (2009) “Light, Sleep, and Circadian Rhythms: Together Again.” PLOS Biology. June 23, 2009 http://dx.doi.org/10.1371/journal.pbio.100014 

54. Zhang, T. et al. (May 2016) “ON and OFF retinal ganglion cells differentially regulate serotonergic and GABAergic activity in the dorsal raphe nucleus. Scientific Reports. 6, 26060; doi: 10.1038/srep26060

55. Blask David. (2003). “Melatonin, Chronobiology, and Cancer.” The National Cancer Institute’s Office of cancer Complementary and Alternative Medicine.   http://cam.cancer.gov/cam/attachments/MelatoninSummary.pdf

56. Ghazaleh Valipour, Parvane Saneei, and Ahmad Esmaillzadeh. (2014) “Serum Vitamin D Levels in Relation to Schizophrenia: A Systematic Review and Meta-Analysis of Observational Studies.” CJEM. July 22, 2014. doi: http://dx.doi.org/10.1210/jc.2014-1887

57. Sun Safety Alliance. “When and where are UV rays strongest?” http://www.sunsafetyalliance.org/uv_rays.html

58. Solis, Michele. (2014) “Born at risk: Prevention in the womb.” Nature. 508, S12–S13 (03 April 2014) doi:10.1038/508S12a http://www.nature.com/nature/journal/v508/n7494_supp/box/508S12a_BX1.html

59. Hobday, Richard. (2000) The Healing Sun. Findhorn Press. P 55-80.

60. Arns M, van der Heijden KB, Arnold LE, Kenemans JL. (2103) “Geographic variation in the prevalence of attention-deficit/hyperactivity disorder: the sunny perspective.” Biological Psychatry. 2013 Oct 15;74(8):585-90. doi: 10.1016/j.biopsych.2013.02.010. http://www.ncbi.nlm.nih.gov/pubmed/23523340

61. Liu D. et al. (2014). “UVA Irradiation of Human Skin Vasodilates Arterial Vasculature and Lowers Blood Pressure Independently of Nitric Oxide Synthase.” Journal of Investigative Dermatology. Volume 134, Issue 7, July 2014, Pages 1839–1846 doi:10.1038/jid.2014.27 http://www.sciencedirect.com/science/article/pii/S0022202X15368780

62. NASA. (May 26, 2016). “Ozone Hole Watch.” http://ozonewatch.gsfc.nasa.gov/

63. HOPES. (April 7, 2013). “Free Radical Damage.” http://web.stanford.edu/group/hopes/cgi-bin/hopes_test/vitamin-d3/

64. Schorr, Melanie et al. (July 14, 2015) “Cortisol Measures Across the Weight Spectrum.” JCEM.

doi: http://dx.doi.org/10.1210/JC.2015-2078

65. Patient. (September 6, 2013) “Pineal Gland and Circadian Rhythms.” http://patient.info/doctor/pineal-gland-and-circadian-rhythms

66. Denissen, J.J.A.; Butalid, Ligaya; Penke, Lars; van Aken, Marcel A. G. (2008). “The effects of weather on daily mood: A multilevel approach”. Emotion, 8, 662-667

http://www.pubfacts.com/detail/18837616/The-effects-of-weather-on-daily-mood-a-multilevel-approach 

67. Harward Health Publications. (September 1, 2008) “Time for more vitamin D.” http://www.health.harvard.edu/newsletter_article/Time_for_more_vitamin_D 

68. Kevin G. McGuigana, Ronán M. Conroya et al. (2012) “Solar water disinfection (SODIS): A review from bench-top to roof-top.“ Journal of Hazardous Materials, Volumes 235–236, 15 October 2012, Pages 29–46doi:10.1016/j.jhazmat.2012.07.053

http://www.sciencedirect.com/science/article/pii/S0304389412007960

69. Intro to Astronomy. (2002) “The sun´s structure and nuclear fusion.” http://earthguide.ucsd.edu/virtualmuseum/ita/07_2.shtml

70. Weller R.B. (2016). “Sunlight Has Cardiovascular Benefits Independently of Vitamin D.” Blood Purification. 2016;41:130-134    doi:10.1159/000441266 http://www.karger.com/Article/FullText/441266

71. Lucas, Robin et.al. 2006. ,,Global burden of disease from solar ultraviolet radiation." Environmental burden of Disease series No. 13. WHO. http://apps.who.int/iris/bitstream/10665/43505/1/9241594403_eng.pdf?ua=1