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Light as medicine

Have you ever wondered why sunbathing makes you feel so relaxed? Or why you get sick more often in winter? A growing number of studies indicate that insufficient sun exposure is bad for our general health. Recent research has found an inverse relationship between the amount of sun exposure and all-cause mortality – that is, very low levels of sun exposure are associated with a significantly higher risk of death from many different diseases [1]. It seems that in our rush to embrace modern lighting technologies the adverse impacts from changing our light environment may have been overlooked. We’re now beginning to understand the powerful biological reasons why getting enough sunlight is so important.


Legs and feet of a person sunbathing on the beach

For millions of years humans have been bathed in sunlight during day and in light from campfires at night [2]. Now, most of us spend our days indoors under lighting that is very different to natural light [2] and we’re only just beginning to understand the consequences. We have long known that sunlight is essential for good health because UV light from the sun kills pathogens on the skin and makes our bodies produce endorphins (feel-good molecules), serotonin and vitamin D [3], an essential immune system modulator. But although higher vitamin D levels in blood are associated with a lower disease burden, studies investigating the benefits of vitamin D supplementation have been disappointing, indicating that there is more to this story [1, 4].


To start let’s get just a little technical. The light and heat energy from the sun is known as solar radiation and is categorized by specific wavelengths. Although sunlight appears white that is because it’s composed of every individual colour which can be observed when the light is bent through a prism, or in a rainbow. But light energy continues in both directions beyond the visible colours. Shorter wavelengths of light energy at one end of the spectrum (beyond blue or violet) are referred to as ultraviolet and the longer wavelengths at the other end (beyond red) are referred to as infrared. Just into the infrared on the solar spectrum comes near-infrared radiation (NIR), and this part of spectrum seems to be particularly important for good health. NIR includes photons (particles of light) with wavelengths from 650nm to 1200nm which actually make up the majority of the photons present in sunlight.


When near-infrared radiation photons hit your skin they convert light into signals that stimulate biological processes.

When NIR photons hit your skin they can travel more than 8 cm into your body, reaching about 60% of your cells [2], and convert light into signals that stimulate biological processes [5]. For example, the absorption of NIR by blood vessels stimulates the release of nitric oxide, which dilates blood vessels [2] and increases blood flow. NIR even penetrates through the skull, and our brains and bodies appear to be optimised to distribute NIR photons using light-guiding effects [2]. You can observe how well NIR enters the body for yourself. Use a flashlight or the light on your phone in a darkened room and place your thumb or fingers over it. The blue and green photons are absorbed and are no longer visible, while the NIR penetrates the thumb and illuminates it [2], making blood vessels visible. This ability of NIR to go deep into our bodies is used in some types of medical imaging.


In addition to increased energy, NIR treatment increases the blood volume and amount of oxygen in blood at the treatment site.

What NIR photons do when they reach our cells has been more difficult to determine, but is slowly coming to light. One thing that NIR does is stimulate the mitochondria, the powerhouses of our cells, increasing the amount of energy they make [6]. In addition to increased energy, NIR treatment increases the blood volume and amount of oxygen in blood at the treatment site [6]. These effects are likely to be very beneficial in wound healing, and may help to counteract the hypoxia (lack of oxygen) created by inflammation and damage to blood vessels from surgery. Hypoxia is a powerful driver of arthrofibrosis.


However, the benefits of NIR go even further. In response to NIR stimulation the mitochondria in our cells produce a powerful antioxidant called melatonin [2]. This protective response acts to prevent damage from oxygen free radicals that mitochondria inevitably create as a normal part of energy production and also from exposure to light. Melatonin is a highly effective free radical scavenger and because it’s produced right where the action is, in the mitochondria, it immediately mops up the dangerous free radicals, quenching the sparks before they create a firestorm of destruction [2]. This reduction in oxidative stress is thought to be one reason why NIR therapy is an effective treatment for wound healing, PTSD, macular degeneration and other conditions [2, 7]. But there is a catch – to get this protection we need exposure to NIR. Modern energy efficient lighting doesn’t have any and energy efficient glass blocks NIR from the sun [2].

Figure from Zimmerman & Reiter 2019 [2]: comparison between artificial blue light and blue light in nature.
Figure from Zimmerman & Reiter 2019 [2]: comparison between artificial blue light and blue light in nature.

Sunlight is the easiest way to obtain whole body NIR in the proportions that we are adapted to, and sun exposure has demonstrated a diverse range of health benefits from reducing inflammation, stress reduction and improvements in symptoms of diabetes [4], depression and fatigue. But unless you live in the tropics it’s unlikely that you will spend enough time outdoors (in lightweight clothing) in the winter months for a sufficient dose. This is where modern technology can help, in the form of photobiomodulation, a rapidly expanding field of research. A recent review of the literature indicated that photobiomodulation using lamps or lasers at specific wavelenghts can reduce inflammation and pain in muskeloskeletal conditions [8], including post-operative pain and swelling [7] and in osteoarthritis [9]. Inflammatory cytokines (proteins the body makes) induce pain by directly activating sensory nerves [7] so by reducing inflammation NIR also reduces pain and oxidative stress in tissues, which in turn reduces further inflammation and fibrosis. These anti-inflammatory, anti-pain and anti-swelling properties of photobiomodulation are now widely accepted, along with increased energy efficiency and enhanced wound healing [7, 10]. This has led photobiomodulation to be used to treat a broad range of conditions [8], including cancer care complications such as tissue fibrosis, and conditions affecting the brain [11].


The ideal parameters for treating fibrosis are not known yet, however, NIR wavelengths between 780 to 950nm are the most commonly used for deep tissue therapy [7, 12], and would likely be necessary for treating large joints. Wavelengths shorter than this are sometimes used for surface treatments. One study reported that NIR at 808nm for 20 minutes together with melatonin supplements improved the healing rates of bones [2]. It should be noted, however, that it will likely take time to notice the positive effects of NIR therapy, manifesting as a gradual change for the better rather than a sudden change as you might experience from an opioid pain killer.


A confusing aspect of photobiomodulation is that studies consistently demonstrate an increased release of latent TGF-β1, the key fibrotic cytokine (protein the body makes), from tissue stores [8]. It has been suggested that this temporary spike in TGF-β1 is necessary to activate the body’s natural “off switch” and turn down TGF-β1 signalling [8]. The complexity of TGF-β1 production, activation, and signalling is emphasised by research showing that global inhibition of TGF-β1 is not effective for treating fibrosis [13]. In any case, photobiomodulation has been shown to consistently reduce inflammation, allowing myofibroblasts to do their job of healthy wound healing and then de-activate [12]. While many photobiomodulation studies are high quality, randomised and placebo controlled [7], there are many methods of delivering photobiomodulation including different durations, frequency of therapy, power output [9] and source of light (LEDs or lasers), complicating an understanding of the effects.


Natural sunlight is free and because the NIR wavelengths are good at going through hats and clothing (if the clothes are not insulated) and even sunblock [2] we can be sun-safe while we’re outdoors and still get the health benefits. And NIR is present in early morning and late afternoon sunlight when UV is greatly reduced. Plants are also highly reflective of NIR so even shade in a green space is rich in NIR [2]. Broadly speaking, the more time spent outdoors the better, and being outdoors in green spaces appears to have many important health benefits [1, 4]. However, sunbathing needs to be carefully timed to avoid sunburn with exposure time adjusted according to your skin type, time of the year and latitude. It pays to play it safe to make sure that you don’t get sunburnt, which itself can lead to increased inflammation as well as skin cancer risk. While for many years we have had a messaging focus on the downsides of sunburn unfortunately the beneficial effects on health of non-burning sun exposure have been largely overlooked.


Artificial NIR from lamps with light-emitting diodes (LEDs) doesn’t provide the relaxing, meditative aspect of sunbathing outdoors in nature, but it can make the process easier in difficult climates or settings. Heating of the treated area should be avoided [8, 12], so complying with the instructions for your device, including the distance from lamp to the treated area, is important. If you decide to supplement with an artificial light, note NIR is not visible in a well lit room, so if you can see a lot of strong red light the lamp may or may not be emitting in the NIR range. Check the specifications and make sure it is from a reputable manufacturer.


This blog was inspired by the excellent and informative videos by Medcram on this topic. Medcram have several videos that clearly explain the health benefits of NIR, including this one.


References

  1. Lindqvist, P. G. The Winding Path Towards an Inverse Relationship Between Sun Exposure and All-cause Mortality. Anticancer Res 38, 1173-1178 (2018). https://doi.org/10.21873/anticanres.12337

  2. Zimmerman, S. & Reiter, R. J. Melatonin and the Optics of the Human Body. Melatonin Research 2, 138-160 (2019). https://doi.org/10.32794/mr11250016

  3. Gimenez, M. C. et al. Effects of Near-Infrared Light on Well-Being and Health in Human Subjects with Mild Sleep-Related Complaints: A Double-Blind, Randomized, Placebo-Controlled Study. Biology (Basel)12 (2022). https://doi.org/10.3390/biology12010060

  4. Shore-Lorenti, C. et al. Shining the light on Sunshine: a systematic review of the influence of sun exposure on type 2 diabetes mellitus-related outcomes. Clin Endocrinol (Oxf) 81, 799-811 (2014). https://doi.org/10.1111/cen.12567

  5. Yamada, E. F. et al. Photobiomodulation therapy in knee osteoarthritis reduces oxidative stress and inflammatory cytokines in rats. J Biophotonics 13, e201900204 (2020). https://doi.org/10.1002/jbio.201900204

  6. Wang, X., Tian, F., Soni, S. S., Gonzalez-Lima, F. & Liu, H. Interplay between up-regulation of cytochrome-c-oxidase and hemoglobin oxygenation induced by near-infrared laser. Sci Rep 6, 30540 (2016). https://doi.org/10.1038/srep30540

  7. Gonzalez-Muñoz, A. et al. Efficacy of Photobiomodulation Therapy in the Treatment of Pain and Inflammation: A Literature Review. Healthcare 11, 938 (2023). https://doi.org/10.3390/healthcare11070938

  8. Arany, P. R. Photobiomodulation-Activated Latent Transforming Growth Factor-beta1: A Critical Clinical Therapeutic Pathway and an Endogenous Optogenetic Tool for Discovery. Photobiomodul Photomed Laser Surg 40, 136-147 (2022). https://doi.org/10.1089/photob.2021.0109

  9. Wyszynska, J. & Bal-Bochenska, M. Efficacy of High-Intensity Laser Therapy in Treating Knee Osteoarthritis: A First Systematic Review. Photomed Laser Surg 36, 343-353 (2018). https://doi.org/10.1089/pho.2017.4425

  10. Genah, S. et al. Effect of NIR Laser Therapy by MLS-MiS Source on Fibroblast Activation by Inflammatory Cytokines in Relation to Wound Healing. Biomedicines 9 (2021). https://doi.org/10.3390/biomedicines9030307

  11. Bowen, R. & Arany, P. R. Use of either transcranial or whole-body photobiomodulation treatments improves COVID-19 brain fog. J Biophotonics 16, e202200391 (2023). https://doi.org/10.1002/jbio.202200391

  12. Khan, I. et al. Accelerated burn wound healing with photobiomodulation therapy involves activation of endogenous latent TGF-beta1. Sci Rep 11, 13371 (2021). https://doi.org/10.1038/s41598-021-92650-w

  13. Murphy-Ullrich, J. E. & Suto, M. J. Thrombospondin-1 regulation of latent TGF-beta activation: A therapeutic target for fibrotic disease. Matrix Biol68-69, 28-43 (2018). https://doi.org/10.1016/j.matbio.2017.12.009

  14. Twohig-Bennett, C. & Jones, A. The health benefits of the great outdoors: A systematic review and meta-analysis of greenspace exposure and health outcomes. Environ Res 166, 628-637 (2018). https://doi.org/10.1016/j.envres.2018.06.030

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