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2/11/20

Basics of Blue Light

Circadian rhythms run our physiology, from the central role of the suprachiasmatic nucleus (SCN) in arousal and sleep cycles to the more obscure peripheral oscillators that depend on coherent synchronization with the master clock. There are four main agents that affect rhythmicity: Light, hormones, temperature, and feeding. This paper will focus on how one agent, light, entrains and affects all of human physiology. The retina mediates these processes via the retinohypothalamic tract, which facilitates autonomic responses and hormonal communication. Literally every cell depends on the rhythm set by the SCN in order to function in their own circadian manner, so it requires little imagination to fathom the extent of dysregulation that can occur with inappropriate phase shifting as a result of prolonged exposure to irritating wavelengths of light. Blue light has somewhat of a paradoxical reputation, with some research praising it for melatonin suppression and arousal promoting effects while others demonize it due to its phototoxicity and insomniogenic attributes. So what are we to make of the literature? Well, just as anything else, moderation is key.

There needs to be an understanding of what is meant by ‘blue light’, as there is a decent chunk of the visible light spectrum devoted to it, from 380 to 500nm. As the literature suggests, blue light (and some of the UV spectrum) is absolutely necessary for proper function. Photoreceptors capture photons from light and utilize the energy to induce a cascade that results in an action - from color vision and dim-light vision to photoentrainment and hormonal periodicity. However, blue light is never on its own in the solar spectrum, and it is vastly outshined by the amount of red and infrared (IR) light radiated along side it. Left alone, blue light has been shown to induce photochemical damage across the retina and macula due to its high-energy photons causing oxidative stress within the opsins. Oxidative stress results in the production of reactive molecular species that damage cell structure and energy flux. Red light, on the other hand, has been shown to play a vital role in cell function and regeneration. In fact, there is a copious amount of literature on the benefits and physiology of low-level laser therapy - all this to say that the divorce of the solar spectrum is wreaking havoc on human physiology at the cellular level. Essentially, there are two substantial issues with man-made light – one being the frequencies (and lack thereof) and two being the duration of exposure. As previously mentioned blue light is not the enemy, however, when it is devoid of its sister spectrum and screens dominate our attention the photic barrage becomes too great for the retina to handle - reactive molecular species will be generated at a higher rate, the ATP yield of mitochondria will be reduced by altered redox potential, energy transfer will be slowed in the absence of IR light, and so on. Understanding things on such a cellular level really helps to grasp effects on global physiology.

Information from light is propagated from the retina to the entire body, which instructs the end organs to work and perform accordingly. One of the greatest arenas that this relationship is so clinically relevant is sleep, where hormonal modulators depend on proper lighting and circadian synchrony. The job of the blue frequency is to initiate a “daytime” command, which assists with bringing along the behaviors needed to operate and function during the day. What happens when we expose our eyes to this during evening hours? Well the brain is told that it is still daytime and that it needs to adapt. The sequela of poor or inadequate sleep includes altered cognition, impaired memory consolidation, problems concentrating, compromised tissue repair, and prolonged sympathetic nervous system activation. All of these issues have their own sequela, which can lead to a laundry list of other symptoms not originally associated with a lack of sleep. While blue light is not the only offender when it comes to disrupted sleep, it is important to recognize that the etiology of much of the population’s disturbed sleep may be prolonged exposure to this zeitgerber.

Although not an effect of blue light directly, another clinical diagnosis that is relevant to the overutilization of digital devices primarily emitting blue light is digital eyestrain. Fatigue of the extraocular eye muscles (EOM’s) can lead to intense symptomatology in addition to that of the metabolic cascade described earlier. All muscles have proprioceptors that give continuous information to the brain regarding position and tension and the change in those parameters. When a muscle is chronically lengthened or shortened, the proprioceptors adapt and this results in tone that is abnormal and dysfunctional. This process occurs within the EOM’s as well and due to the poor posturing associated with the use of computers and mobile devices the EOM’s are subject to vertical, horizontal, and torsional stress. Our eyes and visual systems are built to explore the visual environment, track objects of interest, scan for new objects of interest, and also to search and react to possible threats to safety – all of which occur beyond the measurements of any screen. With blue light causing physical damage to photoreceptors, contrast sensitivity can also take a significant hit, which will reduce visual clarity as well as increase cortical energy expenditure as it devotes more effort to attain accurate perception.

From a basic overview one can appreciate both the benefits and detriments of blue light, while also acknowledging that moderation is likely key to adapting to a world that is so digitally entrenched. The importance of taking steps to limit blue light exposure is great, as is the need to reduce prolonged and unnecessary screen time; however, of greater importance is to spend more time outdoors and within the complete solar spectrum.

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