By Adriana Ramos
Light-emitting diodes (LEDs), used in computers, tablets, smartphones and TVs, have experienced a revolution in the last couple decades. It is important to bear in mind that although the light they emit might seem white, their peak emission is in the blue light range (Tosini, Ferguson and Tsubota, 2016). Wavelengths between 300 and 400 nm can pass through the cornea and be absorbed by the pupil or the iris. As a consequence, within the visible light spectrum, high energy blue light with a wavelength between 415 nm and 455 nm is greatly related to eye diseases, including age-related macular degeneration, cataract and dry eye, among others. Moreover, this type of light can stimulate the brain, enhancing adrenocortical hormone production and inhibiting melatonin secretion, leading to a disruption of hormonal balance and directly affecting sleep patterns and quality (Zhao et al, 2018).
Photoreception, in mammals, occurs in the retina and is mediated by three types of photoreceptors: rods and cones which are mostly responsible for the image-forming (IF) vision, and intrinsically photosensitive retinal ganglion cells (ipRGCs) that are vital for the non-image-forming (NIF) photoreception as well as responsible for the regulation of pupillary light response and circadian photic entrainment, among other functions. In animals and blind human patients with completely degenerated rod/cone photoreceptors, NIF responses were recorded almost intact, suggesting that the adaptation of physiology and behavior to light is independent from vision (Hatori et al., 2017). Retinal ganglion cells express PACAP (pituitary adenylate cyclase-activating polypeptide) and form the retinohypothalamic tract (RHT), which conveys the light information from the retinal ganglion cells to the brain area that controls circadian rhythms. Only the RGCs that express melanopsin (the most likely circadian retinal photo pigment), are considered ipRGCs, and these cells were no longer intrinsically photosensitive in melanopsin knockout (KO) mice (even though cell number, projections, and morphology remained unchanged) (Tosini et al, 2016). The NIF responses were not abolished but attenuated in this type of mouse and were completely eliminated in mice that lacked both melanopsin and functional outer retina photoreceptors (Hatori et al., 2017). The non-visual stimulus, once detected by the ipRGCs, is transmitted directly to the suprachiasmatic nucleus (SCN) of the hypothalamus (the master circadian clock responsible for organizing the recurring physiological functions such as hormone secretion and body temperature). The light-dark transitions perceived by the eyes synchronize the master circadian clock, though it can be disturbed by artificial light-caused changes in the light-darkness pattern, especially at night. It has been estimated that there has been an annual 3-6% increase in exposure to artificial light at night (ALAN) in the last 15-20 years (Tähkämö et al, 2018).
In order to maintain organismal homeostasis in synchrony, circadian rhythms are essential in human metabolism, behavior and physiology (Hatori et al., 2017). A circadian rhythm can be determined by measuring melatonin levels, which is secreted in a cyclic fashion. In other words, melatonin levels are low during daytime and high at night (in the dark); this level of high melatonin secretion, however, depends on age and sex. Several diseases are linked with a disruption in the normal levels of melatonin secretion, most importantly some mental disorders, metabolic syndrome and some types of cancer, but exposure to artificial light has not been proven to cause these conditions. Circadian disruption has recently been classified as a possible factor contributing towards cancer development in humans by the International Agency for Research on Cancer (IARC). This presents the possibility that night-time light exposure might contribute to cancer development, since it can potentially delay (phase-shift) or even suppress melatonin secretion. Circadian rhythms can also be detected by using an electroencephalogram (EEG) to measure REM sleep parameters, inasmuch as rapid-eye-movement (REM) sleep timing is controlled by the SCN and light exposure affects not only melatonin secretion but also sleep-wakefulness cycle (Tähkämö, Partonen and Pesonen, 2018).
To sum up, while the blue-light component of daylight peaks around midday, the constant exposure to LED lights from our multiple devices exposes us to higher levels of this type of light, especially in the evening and at night. While treatments like “bright light therapy” or “phototherapy” use light exposure against depression and circadian sleep disorders, studies carried out my epidemiologists point to a negative association between night-time use of electronic devices and sleep quality and quantity, since it can cause melatonin suppression, circadian phase delay and a reduced cognitive performance (Hatori et al., 2017). Not only is it important to avoid bright screens at night, but also increase the exposure to this blue portion of the artificial light during daytime as it is key to the vitality of the organisms. This puts forward the relevance of the use of a proper, bright artificial lighting with a more blue‐weighted spectrum during daytime, which additionally can lead to an improvement in the health of patients in nursing homes or hospitals and the learning ability and performance of school kids and employees working indoors (Wahl et al., 2019).
Hatori, M., Gronfier, C., Van Gelder, R., Bernstein, P., Carreras, J., Panda, S., Marks, F., Sliney, D., Hunt, C., Hirota, T., Furukawa, T. and Tsubota, K., 2017. Global rise of potential health hazards caused by blue light-induced circadian disruption in modern aging societies. npj Aging and Mechanisms of Disease, 3(1).
Tosini, G., Ferguson, I. and Tsubota, K., 2016. Effects of blue light on the circadian system and eye physiology. Molecular Vision: Biology and Genetics in Vision Research, 22, pp.61-72.
Tähkämö, L., Partonen, T. and Pesonen, A., 2018. Systematic review of light exposure impact on human circadian rhythm. Chronobiology International, 36(2), pp.151-170.
Wahl, S., Engelhardt, M., Schaupp, P., Lappe, C. and Ivanov, I., 2019. The inner clock—Blue light sets the human rhythm. Journal of Biophotonics, 12(12).
Zhao, Z., Zhou, Y., Tan, G. and Li, J., 2018. Research progress about the effect and prevention of blue light on eyes. International Journal of Ophthalmology, 11(12), pp.1999-2003.