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Blue light, photoreceptors and human sleep



Blue light has the ability to stimulate a series of photosensitive cells in the retina that not only allow us to perceive our surroundings but also regulate our sleep (Teran, 2020) and control the pupillary response. In this blog, we will explore in detail these photosensitive cells designed to capture light.


Light and the Electromagnetic Spectrum



Figure 1. Solar radiation spectrum and a black body.

Visible light, crucial to our vision, constitutes only a tiny fraction of the total electromagnetic spectrum. This spectrum encompasses the energy emanating from light sources such as the sun and is distributed based on wavelength or frequency.


The visible light spectrum is located between 400 and 700 nanometers, fitting between ultraviolet light and infrared light. Ultraviolet light, with wavelengths between 280 and 400 nanometers, is known for its potential to damage tissues due to its high energy. On the other hand, infrared light, despite having less energy due to its longer wavelength, interacts intensely with our tissues, resulting in heat.


Blue light, which resides within the visible light spectrum, is important to note. This short wavelength range is known to have a significant impact on our health, especially in the regulation of sleep. Although some studies have suggested that blue light can have negative effects on the retina, these results are still being evaluated and have not been conclusively proven.


Photons, particles that carry the sun's light energy, are captured by an intricate network of photosensitive cells in our eyes. Through absorption processes, these cells transform solar energy into bioelectric signals that our brain can interpret, thus allowing vision. We will delve into how these photoreceptors work and the specific influence of blue light on their operation.


Classic Photoreceptors: Cones and Rods


The classic photoreceptors in the human eye are known as cones and rods. The cones, of which there are three types, have varying sensitivities to different regions of the visible light spectrum and are responsible for photopic vision, or color vision.


L-type cones, also known as red cones, primarily respond to longer wavelengths, greater than 560 nanometers. M-type cones, or green cones, are sensitive to intermediate wavelengths, approximately around 530 nanometers. Finally, S-type cones, or blue cones, are more sensitive to shorter wavelengths, around 430 nanometers.


Each type of cone in our eyes harbors specific pigments designed to absorb and capture light at different wavelength ranges. L cones contain a pigment known as erythropsin, while M cones incorporate chloropsin. S cones, on the other hand, are equipped with cyanopsin. As can be seen in Figure 2, only the S cone has a peak sensitivity in the short wavelength region, although all are capable of perceiving this range, but with a lower intensity than their respective absorption peaks.


Absorption spectra of red cones (L), green cones (M), and blue cones (S). In addition to the rods (R).

Absorption spectra of red cones (L), green cones (M), and blue cones (S). In addition to the rods (R). On the other hand, we have the rods, photoreceptor cells especially sensitive to low light. They are responsible for scotopic vision, which allows us to see in low light conditions or during the night. Unlike cones, rods are not involved in the perception of color. Instead, they provide us with a vision in grayscale. This is why, in low light conditions, we perceive the world mainly in shades of gray. In addition, rods are more abundant in the periphery of the retina, which explains our better peripheral vision in low light conditions. It is important to note that, although rods are very sensitive to light, they do not respond well to blue light. In general, their peak sensitivity is around 498 nanometers, in the range of bluish-green light.


All these photoreceptors, both cones and rods, make a significant contribution in the short wavelength range, around 480 nanometers. This indicates that they have the ability to interact and regulate the perception of blue light, even though none of them have their peak sensitivity in this range. However, the relationship between blue light and human vision is not limited to the interaction of this wavelength with the classic photoreceptors. Intrinsically photosensitive retinal ganglion cells (ipRGCs), represent another essential component in this interaction.


Intrinsically Photosensitive Retinal Cells: Importance in Sleep Regulation


Intrinsically photosensitive retinal ganglion cells (IPRGCs) are specialized photoreceptors that play a key role in regulating our circadian rhythms (Tosini, 2016). There are several subtypes of IPRGCs, identified as M1 to M5. All are sensitive to light, but their sensitivity varies depending on the wavelength.


M1 cells are particularly sensitive to blue light (wavelength of approximately 480 nanometers). This subtype of IPRGC is crucial in the regulation of our circadian cycle. When blue light hits M1 cells, it stimulates the production of a protein called melanopsin. Melanopsin, in turn, sends signals to the suprachiasmatic nucleus (SCN) of the brain, which is the main regulator of our circadian rhythms.


When the SCN receives the signal that it is day (i.e., when light is detected), it inhibits the production of melatonin, a hormone that promotes sleep. Conversely, when light decreases or goes away, the SCN allows the production of melatonin, which helps us feel drowsy and prepares our body for rest.


Therefore, exposure to light, particularly blue light, can have a significant impact on our sleep patterns. During night hours, exposure to blue light can inhibit the production of melatonin and disrupt our circadian rhythms. This is a fact that has become especially relevant in our modern society, where exposure to blue light from the use of electronic devices is common during the night. However, not all IPRGCs are only tasked with transmitting light information to circadian control centers. Some subtypes, such as M2 and M4 cells, can also send signals to the pretectal area of the brain to control the pupillary light reflex. Others, like M5 cells, may be involved in contrast perception.


In summary, IPRGCs are crucial for a range of functions from sleep regulation to adaptation of our pupils to light conditions. Although much has been advanced in our understanding of these cells, there is still much to learn about their various functions and how they interact with other photoreceptors in the retina.


Final Remarks


The study of blue light and its interaction with the visual system is essential to understand its role in vital processes like sleep regulation. Blue light influences the production of melatonin, a key component in our sleep-wake patterns.


Sleep is essential, not only as an energy recharging process but also as a critical phase in which our brain performs maintenance tasks, including the elimination of neuronal waste produced during daytime activity. This brain cleanup is vital to keep our cognitive functions in optimal conditions. Depriving ourselves of sleep can generate a harmful accumulation of these wastes, triggering symptoms such as fatigue, tiredness, and difficulties concentrating. Furthermore, chronic sleep disruption can alter mood, impair decision-making, and affect long-term memory. Therefore, understanding the importance of quality sleep for our brain health and overall well-being is crucial. Within this framework, blue light plays a significant role in regulating our sleep patterns, underlining the need to manage our exposure to it appropriately.


Controlling exposure to blue light, especially before sleep, can help improve the quality of our sleep. In summary, blue light has a profound impact on our health and well-being, beyond its role in vision. As we delve deeper into its study, we have the opportunity to improve our health and quality of life.


Emiliano Teran


References


Teran E, Yee-Rendon CM, Ortega-Salazar J, De Gracia P, Garcia-Romo E, Woods RL. Evaluation of Two Strategies for Alleviating the Impact on the Circadian Cycle of Smartphone Screens. Optom Vis Sci. 2020 Mar;97(3):207-217. doi: 10.1097/OPX.0000000000001485.

Tosini G, Ferguson I, Tsubota K. Effects of blue light on the circadian system and eye physiology. Mol Vis. 2016 Jan 24;22:61-72. doi: 10.1097/OPX.0000000000001866.





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