@ Sydney Bright

When pondering human health, it is essential to consider what an ancient hominid living in nature experienced. The daily lifestyle, everyday activities, and frequent exposures can all be helpful guides in helping us understand what is healthy or unhealthy in modern life. Though we live in technological complexity, our bodies evolved to adapt to the simple rhythms of nature. Given all we have gained, it can be easy to take what we have lost in our modern life for granted. One example is our routine exposure to the cycle of light and darkness. Our circadian rhythm plays a crucial role in our health. Disruptions to the circadian rhythm brought about by light exposure during nighttime have been associated with bipolar disorder, anorexia, breast cancer, obesity, high blood pressure, and coronary heart disease1–6. However, these experimental observations may only be scratching the surface of the importance a healthy rhythm of light exposure may have on our health. Light interacts with our bodies in two main ways: our eyes and skin. Readers who may be familiar with basic human physiology would likely be aware that our eyes detect light via the rod and cone photoreceptors. However, many may be unfamiliar with a third photoreceptor, melanopsin-containing intrinsically photosensitive retinal ganglion cells7. The name is certainly a handful, so they can also be called ipRGCs. These photoreceptors do not inform our visual senses but detect light and subsequently activate the suprachiasmatic nucleus (SCN) of the hypothalamus8. The stimulus of the SCN results in a signalling cascade to the paraventricular nucleus of the hypothalamus, then to the pineal gland, which metabolizes melatonin5. In this way, the SCN acts as the master circadian clock that organizes daily reoccurring functions related to light, such as hormone secretion and body temperature9. Therefore, circadian clock synchronization occurs via light and dark transitions, and disruption to this natural rhythm can impair normal physiologic functions9. A list of possibly associated diseases caused by such disruption has already been listed. However, it would be more appropriate to describe the consequences of this disruption as a cascade of health dysfunction. For example, a study was conducted on mice where pregnant mice were kept under constant light exposure10. As a result, the mother’s reproductive system resulted in impaired ovarian follicle development, reduced oocyte quality, and preimplantation embryo development. The offspring were also affected, resulting in greater obesity rates, abnormal lipid metabolism, and early puberty onset10. These alarming findings are not limited to mice studies either. A study on humans was conducted where individuals exposed to low light levels during sleep were monitored. The study found that low light exposure levels during sleep resulted in less REM sleep, higher heart rate, and lower heart rate variability11. Heart rate variability is usually a proxy measurement to determine vagus nerve activation, which indicates high parasympathetic nervous system (PNS) activity12. The counteracting system to the PNS is the sympathetic nervous system (SNS), more commonly known as the system responsible for the “flight or fight” response. In this study, higher SNS activity correlated with more significant insulin and insulin resistance11. This indicates the potential mechanism of why circadian rhythm disruption causes weight gain and is also a likely explanation for why fixing someone’s circadian rhythm is beneficial in treating major depressive disorder13. Not only is exposure to light at night unhelpful, but there are many benefits to exposure to light during the day. Firstly, many health benefits are directed toward the eyes when exposed to long wavelength light of about 650-900nm, which occurs during sunrise or sunset. The retina of the eyes has the greatest metabolic demand of the body and the greatest amount of mitochondrial density14. When the retina is exposed to this morning (or sunset) light, mitochondrial function improves, and a significant increase in ATP (the body’s “fuel source”) production occurs14,15. Improved mitochondrial function reduces the rate of age-related photoreceptor loss in the eyes16. Additionally, morning light in the eyes also increases cytochrome c oxidase (COX), the enzyme that regulates oxidative phosphorylation, which correlates with reduced component CS, an inflammatory marker in the retina17. This means exposure to this kind of light significantly improves and prolongs the health of our eyes. As previously mentioned, the benefit of light exposure is not limited to when light enters the eyes. Imagine that when we lived out in nature, it was only natural for our eyes to be exposed to the morning light, and we likely enjoyed the sunset view at the end of the day. Our skin was also constantly being exposed to sunlight throughout the day. UVB light, which is most abundant during the middle of the day, provides many physiologic benefits when exposed to skin. When the back of mice is exposed to UVB, several hormones related to the HPA axis are released, such as corticotropin-releasing hormone (CRH), β-endorphin, and adrenocorticotropic hormone (ACTH)18. It also results in the inhibition of IL-10, leading to an immunosuppressive effect18. Interestingly, UVB also seems to trigger a sexual response in mice, enhancing sexual responsiveness, attractiveness, and interactions among males and females in a laboratory setting19. Due to the benefits of light exposure to our skin, some research has also been directed to finding therapeutic use of LED light. For example, LED treatment can lead to a 52% acne reduction if performed every other day for 8 weeks20. Research has also shown that it has therapeutic benefits for healing herpes20. Our modern, technically advanced world is full of great wonder and luxury. However, we should not be willfully ignorant of the physiologic needs of our bodies. Some may like to pretend that we can live in total seclusion from nature and be fully immersed in a world of technology. The truth, however, is that our bodies are structured to thrive in nature’s natural rhythms. Being exposed to light during naturally dark times can burden our health. At the same time, exposing our bodies to light when nature provides it can allow our bodies to function optimally. Though we are diligently building a technological fortress, we cannot forget the nature of our first home. References 1. Sun S, Cao W, Ge Y, et al. Outdoor light at night and risk of coronary heart disease among older adults: a prospective cohort study. Eur Heart J. 2021;42(8):822-830. doi:10.1093/EURHEARTJ/EHAA846 2. Park YMM, White AJ, Jackson CL, Weinberg CR, Sandler DP. Association of Exposure to Artificial Light at Night While Sleeping With Risk of Obesity in Women. JAMA Intern Med. 2019;179(8):1061-1071. doi:10.1001/JAMAINTERNMED.2019.0571 3. Rybnikova N, Haim A, Portnov BA. Artificial Light at Night (ALAN) and breast cancer incidence worldwide: A revisit of earlier findings with analysis of current trends. Chronobiol Int. 2015;32(6):757-773. doi:10.3109/07420528.2015.1043369 4. Esaki Y, Obayashi K, Saeki K, Fujita K, Iwata N, Kitajima T. Habitual light exposure and circadian activity rhythms in bipolar disorder: A cross-sectional analysis of the APPLE cohort. J Affect Disord. 2023;323:762-769. doi:10.1016/J.JAD.2022.12.034 5. Gabloffsky T, Gill S, Staffeld A, et al. Food Restriction in Mice Induces Food-Anticipatory Activity and Circadian-Rhythm-Related Activity Changes. Nutrients. 2022;14(24):5252. doi:10.3390/NU14245252/S1 6. Shen M, Li Y, Li S, Chen X, Zou B, Lu Y. Association of exposure to artificial light at night during adolescence with blood pressure in early adulthood. Chronobiol Int. 2023;40(10):1419-1426. doi:10.1080/07420528.2023.2266485 7. Spitschan M. Time-Varying Light Exposure in Chronobiology and Sleep Research Experiments. Front Neurol. 2021;12:654158. doi:10.3389/FNEUR.2021.654158/BIBTEX 8. Hannibal J, Hindersson P, Østergaard J, et al. Melanopsin Is Expressed in PACAP-Containing Retinal Ganglion Cells of the Human Retinohypothalamic Tract. Invest Ophthalmol Vis Sci. 2004;45(11):4202-4209. doi:10.1167/IOVS.04-0313 9. Tähkämö L, Partonen T, Pesonen AK. Systematic review of light exposure impact on human circadian rhythm. Chronobiol Int. 2019;36(2):151-170. doi:10.1080/07420528.2018.1527773 10. Guan Y, Xu M, Zhang Z, et al. Maternal circadian disruption before pregnancy impairs the ovarian function of female offspring in mice. Science of The Total Environment. 2023;864:161161. doi:10.1016/J.SCITOTENV.2022.161161 11. Mason IC, Grimaldi D, Reid KJ, et al. Light exposure during sleep impairs cardiometabolic function. Proc Natl Acad Sci U S A. 2022;119(12):e2113290119. doi:10.1073/PNAS.2113290119/SUPPL_FILE/PNAS.2113290119.SAPP.PDF 12. Laborde S, Mosley E, Thayer JF. Heart rate variability and cardiac vagal tone in psychophysiological research - Recommendations for experiment planning, data analysis, and data reporting. Front Psychol. 2017;8(FEB):213. doi:10.3389/fpsyg.2017.00213 13. Xavier NB, Abreu ACVO, Amando GR, et al. Chronobiological parameters as predictors of early treatment response in major depression. J Affect Disord. 2023;323:679-688. doi:10.1016/J.JAD.2022.12.002 14. Gkotsi D, Begum R, Salt T, et al. Recharging mitochondrial batteries in old eyes. Near infra-red increases ATP. Exp Eye Res. 2014;122:50-53. doi:10.1016/J.EXER.2014.02.023 15. Shinhmar H, Hogg C, Neveu M, Jeffery G. Weeklong improved colour contrasts sensitivity after single 670 nm exposures associated with enhanced mitochondrial function. Scientific Reports 2021 11:1. 2021;11(1):1-9. doi:10.1038/s41598-021-02311-1 16. Sivapathasuntharam C, Sivaprasad S, Hogg C, Jeffery G. Improving mitochondrial function significantly reduces the rate of age related photoreceptor loss. Exp Eye Res. 2019;185:107691. doi:10.1016/J.EXER.2019.107691 17. Begum R, Powner MB, Hudson N, Hogg C, Jeffery G. Treatment with 670 nm Light Up Regulates Cytochrome C Oxidase Expression and Reduces Inflammation in an Age-Related Macular Degeneration Model. PLoS One. 2013;8(2):e57828. doi:10.1371/JOURNAL.PONE.0057828 18. Skobowiat C, Postlethwaite AE, Slominski AT. Skin Exposure to Ultraviolet B Rapidly Activates Systemic Neuroendocrine and Immunosuppressive Responses. Photochem Photobiol. 2017;93(4):1008-1015. doi:10.1111/php.12642 19. Parikh R, Sorek E, Parikh S, et al. Skin exposure to UVB light induces a skin-brain-gonad axis and sexual behavior. Cell Rep. 2021;36(8):109579. doi:10.1016/J.CELREP.2021.109579/ATTACHMENT/8A70CE68-0668-4A9D-AEDE-FFDF1AA974F0/MMC5.XLSX 20. Jagdeo J, Austin E, Mamalis A, Wong C, Ho D, Siegel DM. Light-emitting diodes in dermatology: A systematic review of randomized controlled trials. Lasers Surg Med. 2018;50(6):613-628. doi:10.1002/LSM.22791