The benefits of increased exposure to sunlight, both for mental and physical health have been recognised by doctors for centuries, with Hippocrates routinely prescribing ‘sunbaths’ as treatment for a variety of maladies as far back as 400 BC. The Roman philosopher Aulus Cornelius Celsus (25 BC to 50 AD) recommended that sufferers of melancholy live in spaces full of light; and in 1863 Florence Nightingale appealed to hospital designers to include wards that were brightly lit by natural sunlight. In 1984, ‘Seasonal Affective Disorder’ (SAD) was recognised as a condition which causes lethargy and sadness during the winter months when people spend more time indoors and are exposed to less natural light. With our modern technology-based lifestyles, there are now growing concerns that depressive symptoms linked to a deficit in sunlight exposure may no longer be limited to the winter months. Whilst for many people the health benefits of spending more time outside are obvious, researchers have been using scientific methods to investigate the positive impact on mental health and wellbeing of increased exposure to natural light (aan het Rot, Moskowitz & Young, 2006; Genuis, 2006; Walch et al., 2005).
There appear to be a number of factors involved in the positive role sunlight plays in promoting and maintaining mental wellbeing.
Serotonin
Serotonin is a neurotransmitter, commonly associated with feelings of wellbeing and happiness, although its biological function is in fact much more complex, with links to numerous physiological processes, including appetite and digestion, sleep, memory, cognition, reward and learning. Low serotonin levels are often attributed to a number of mental health issues, including anxiety, depression, panic attacks, insomnia, eating disorders, alcohol abuse, low self-esteem, and obsessive thoughts and behaviours. Conversely, higher levels of serotonin correlate with better mood and feelings of satisfaction and calmness. Many antidepressants work by boosting levels of serotonin among brain neurons, and sunlight has been found to have a similar effect.
Studies have measured levels of brain chemicals flowing directly out of the brain and found that people have higher serotonin levels on bright sunny days than they do on cloudy ones, and that effect remains irrespective of temperature.
An Australian study involving 101 healthy men found that levels of serotonin in their brains increased in direct relationship to their exposure to sunlight. Catheters placed in the internal jugular veins of the participants allowed assessments to be done as they were exposed to varying degrees of sunlight. The study found that “the rate of production of serotonin by the brain was directly related to the prevailing duration of bright sunlight, and rose rapidly with increased luminosity” (Lambert, G., Reid, C., Kaye, D., Jennings, G., & Esler, M., 2002).
Vitamin D
Optimum levels of vitamin D are important for overall health and wellbeing (Gillie, 2005). Although vitamin D is found in certain foods, humans get most of their Vitamin D from exposure to sunlight. However, the World Health Organisation estimates that over a billion people are currently deficient in vitamin D (approximately 50% of all Americans) as a result of our modern indoor lives, and considers vitamin D deficiency as a major public health problem worldwide across all age groups. Vitamin D deficiency is linked to a number of health issues, including tiredness, bone and back pain and depression.
Although the amount of sunlight necessary varies according to age, skin type, and diet, the WHO recommends getting 10–30 minutes of sunlight, several times per week in order to maintain healthy blood levels.
Ultraviolet light
Natural light also contains a spectrum of light wavelengths. Although access to sunlight brings the risk of exposure to dangerous levels of ultraviolet (UV) light, recent research also suggests that UV-induced release of nitric oxide from skin may have certain health benefits, including lowering the incidence of hypertension and cardiovascular disease (Liu et al., 2014).
There’s also evidence that UV light can push melanocytes—the cells that produce dark pigment in skin—to release endorphins, a feel-good chemical (Fell et al., 2104).
References
aan het Rot, M., Moskowitz, D., Young, S. (2006). Exposure to bright light is associated with positive social interaction and good mood over short time periods: a naturalistic study in mildly seasonal people. Journal of Psychiatric Research.
Fell, G., Robinson, K., Clifford, J., Woolf, D. & Fisher, E. (2014). Skin β-Endorphin Mediates Addiction to UV Light. Cell,
157(7), 1527-1534.
Genuis, S. J. (2006). Keeping your sunny side up. How sunlight affects health and well-being. Canadian Family Physician, 52(4), 422-3, 429-31.
Gillie, O. (2005). Sunbathing is needed for optimum health in the British Isles. Health Research Forum.
Lambert, G., Reid, C., Kaye, D., Jennings, G., & Esler, M. (2002). Effect of sunlight and season on serotonin turnover in the brain.
The Lancet, 360,1840-42.
Liu, D., Fernandez, B., Hamilton, A., Lang, N., Gallagher, J., Newby, D., Feelisch, M., & Weller, R. (2104). UVA irradiation of human skin vasodilates arterial vasculature and lowers blood pressure independently of nitric oxide synthase. Journal of Investigative Dermatology, 134(7), 1839-46.
Walch, J., Rabin, B., Day R, Williams, J., Choi, K., & Kang, J. (2005). The effect of sunlight on postoperative analgesic medication use: a prospective study of patients undergoing spinal surgery. Psychosomatic Medicine, 67, 156-63.