The Benefits of Sunlight

Summer is upon us again. Sunlight and warm temperatures encourage us to be outside, soaking in those lovely rays, especially after a long, cold, and gray winter. Studies have shown that exposure to sunlight affects many of our behaviors. For example, sleep/wake patterns change as the amount of available sunlight changes, leading to earlier wake-up times and shorter sleep durations in the spring and summer compared to those typically seen in the winter. Both bedtimes and waketimes tend to be slightly later as the temperature outdoors increases (Mattingly, Grover, et al., 2021).

The suprachiasmatic nucleus (SCN) is the central circadian pacemaker. The SCN is located in the hypothalamus and is regulated by light signals from the eye. The SCN then affects a wide variety of physiological and behavioral outcomes

Source: Robin M. Voigt, Ph.D., Christopher B. Forsyth, Ph.D., and Ali Keshavarzian, M.D., Public domain, via Wikimedia Commons

These changes in the patterns of our daily lives, our circadian rhythms, are predictable. The amount of sunlight available in the environment around us sets our biological clock that regulates when we will be up and active and down and asleep. Sunlight also stimulates the synthesis of serotonin, a neurotransmitter known to play an important role in regulations of mood, emotion and arousal. We tend to have higher serotonin levels on bright and sunny days than we do on dark and cloudy ones (Blume, Garbazza and Spitschan, 2019).

Several studies have found that exposure to sunlight also affects both learning and mood. Researchers have discovered a third type of light sensitive receptor cell in the retina, in addition to the familiar rods and cones. These receptors, called intrinsically photosensitive retinal ganglion cells or ipRGC’s, react to light but do not seem to participate in forming a visual image. Instead, they send information about available light to our internal clock (located in a region called the Suprachiasmatic Nucleus or SCN) as well as to other brain regions like the habenula that regulate learning and mood (Fernandez, Fogerson, Ospri, et al., 2018). Kent, McClure, et al., (2009) found that decreased exposure to sunlight tended to increase the probability of cognitive impairment in depressed patients.

Sunlight as disease treatment

A 30 kHz bright light therapy lamp (Innosol Rondo) used to treat seasonal affective disorder. Provides 10,000 lux at a distance of 25 cm.

Source: User:Mysid, Public domain, via Wikimedia Commons

Recognition of the role that light plays in human behavior has led to the development of treatments for sleep and mood disturbances that use exposure to artificial sunlight as treatment. To be effective, light therapy needs to mimic real sunlight. Sunlight consists of packets of energy that travel in waves and is made up of ultraviolet, visible, and infrared light. Visible light is composed of several wavelengths of light, referring to the distance, in nanometers, from peak to adjacent peak in the waves of light. Short wavelengths (380 – 450 nm) are interpreted in our brains as violet or blue light, and longer wavelengths (620 – 750 nm) are perceived as red light, with green and yellow in between. UV light (wavelengths shorter than 400 nm) and infrared light (wavelengths longer than 700 nm) are not visible to the human eye but still have an effect on our bodies and minds, UV light in particular.

Treatment using banks of light that mimic sunlight has been found to be effective in treating Seasonal Affective Disorder (SAD), a form of depression associated with lack of sunlight, as typically seen in the winter months. Light therapy is also being tested as a non-pharmaceutical treatment for non-seasonal depression as well as adult ADHD (Campbell, Miller, and Woesner, 2017).

Exposure to UV light affects the functioning of our immune systems and so our experience of disease. Normally, in a healthy and balanced immune system, T cells (a type of white blood cell) detect invading pathogens and fight them off. Immune systems that are out of balance, either underactive or overactive, lead to disease. Some diseases (cancer for example) can suppress the immune system, making us more susceptible to all disease. Problems like allergies, asthma and eczema result from an overactive immune system. Your immune system can even turn on you and target normal healthy cells, creating autoimmune diseases like type 1 diabetes, Rheumatoid arthritis, lupus and Multiple Sclerosis or MS.

Fisher and Kripke (1977) reported that exposure to UV radiation suppressed the function of the immune system in mice. The same immune system suppression is seen in humans; exposure to UV light produced suppression of normal T-cell responses to cancerous skin cells which is why prolonged or repeated exposure to sunlight increases our risk of skin cancer. However, UV exposure also increases vitamin D production, which boosts the response of the immune system in targeting and destroying pathogens. Results like these led doctors to propose using light therapy to treat diseases that result from immune system imbalance.

Langer-Gould and her colleagues have been examining the use of exposure to UV light to treat the autoimmune disease of MS. Studies in her laboratory have reported that “Lifetime sun exposure appears to reduce the risk of MS regardless of race/ethnicity” (Langer-Gould, Lucas, Xiang, et al., 2018, pg. 1). The effect of sun exposure was also apparently not related to vitamin D levels, hinting at the complexity of our immune response.

While it might seem contradictory, UV light exposure both boosts and suppresses immune system responses. Research on just how much sunlight exposure is needed to increase vitamin D production without risking immune suppression is ongoing (Gonzalez Maglio, Paz and Leoni, 2016), and exploration of how to use light therapy to treat MS and other autoimmune diseases is also progressing.