Shedding light on human health and behaviour
Light is one of the main variables influencing the routine of live organisms on our planet, it is the main source of energy directly or indirectly. Light is part of our lives since we were born. Evolutionary scientists would say that we were born keeping the best features of how to deal with light from our ancestors. In practice: we usually feel happier on a sunny day. That is difficult to deny, and most of us feel sleepy when the darkness comes. We live in a high standard of integration with environmental signals around us, and like temperature, light influences how our body works in a lot of aspects. We can feel things repeating on a daily basis: hunger at lunch time, sleepiness in the evening, even headaches sometimes. The scientific name for this is circadian rhythm: biological processes that display an oscillation of about 24 hours which are reset by exposure to light and darkness. The thing is: Can we follow the biological routine how it should be?
Studies published in the early 1970’s established the suprachiasmatic nucleus of the hypothalamus in the brain as the central circadian pacemaker in mammals [1]. The suprachiasmatic nucleus receives direct input from the retina [2], providing a mechanism by which entrainment to light-dark cycles occurs. Recently, a subset of retinal ganglion cells has been described that serve as photoreceptors for circadian and other non-image-forming responses [3]. There is also evidence that rod and cone photoreceptors can play a role in circadian responses to light [4]. The relative contribution of different photoreceptors to circadian responses is not yet well understood, and this is currently an area of intense research. It is most likely that the intensity, spectral distribution and temporal pattern of light can all affect the relative contribution of different photoreceptors to circadian responses in our body.
Studies published in the 1980’s and 1990’s used body temperature and the cortisol level in plasma as markers of human circadian rhythm and they found that humans, like other organisms, are most sensitive to light stimuli during the biological night, and far less sensitive to light in the middle of the biological day [5]. One of the major players in human day-night rhythm is the hormone melatonin, which is produced during the night and inhibited by light exposure in the day [6]. Many studies show that light exposure during the night is not good for our health. Light at night can increase the risk of breast cancer [7], colon cancer [8], prostate cancer [9], it can reduce immunity [10], it can increase weight, impair cognitive performance, and promote mood disorders [11]. Many studies are evaluating the effect of light therapy during the day, as well as light management in the day-night routine, but for any intervention in your routine at this level your doctor must be consulted.
In general, non-excessive light exposure during the day can be very beneficial for us. And a good “dark” night sleep can decrease the possibility of many diseases related to disturbance of circadian rhythm and as a bonus your mood can be improved.
Resources:
1-Weaver DR. The suprachiasmatic nucleus: a 25-year retrospective. J Biol Rhythms 1998;13(2):100–112.
2-Moore RY, Speh JC & Card JP. The retinohypothalamic tract originates from a distinct subset of retinal ganglion cells. J Comp Neurol 1995;352:351–366.
3-Hattar S, et al. Melanopsin-containing retinal ganglion cells: architecture, projections, and intrinsic photosensitivity. Science 2002;295:1065–1070.
4-Güler AD, et al. Melanopsin cells are the principal conduits for rod-cone input to non-image-forming vision. Nature 2008;453(7191):102–105.
5-Jewett M.E., et al. Human circadian pacemaker is sensitive to light throughout subjective day without evidence of transients. Am J Physiol 1997;273:R1800–R1809.
6- Lewy, A.J., et al. Light suppresses melatonin secretion in humans. Science, 1980, 210(4475), pp.1267-1269.
7- Stevens, R.G.. Light-at-night, circadian disruption and breast cancer: assessment of existing evidence. International journal of epidemiology, 2009, 38(4), pp.963-970.
8- Karantanos T, et al. Clock genes: their role in colorectal cancer. World J. Gastroenterol. 2014, 20, 1986–1992.
9- Zhu Y, et al. Does ‘clock’ matter in prostate cancer? Cancer Epidemiol. Biomark. Prev. 2006, 15, 3–5.
10- Navara, K.J. and Nelson, R.J. The dark side of light at night: physiological, epidemiological, and ecological consequences. Journal of pineal research, 2007, 43(3), pp.215-224.
11- Touitou, Y., Reinberg, A., & Touitou, D. Association between light at night, melatonin secretion, sleep deprivation, and the internal clock: Health impacts and mechanisms of circadian disruption. Life Sciences, 2017, 173, 94–106.
