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There are several environmental time cues that can be used with agents that synchronize the circadian expression of living beings, among them, social interaction, physical activity and food. Even so, there is a consensus that the main synchronizing agent of circadian rhythm is the light/dark cycle. Learn more in the previous text about circadian rhythm

In humans, as among all mammals, the only photoreceptor organ is the retina, a structure present in the eyes that aims at processing light information and thus making vision possible. Thus, it is plausible to imagine that it is through vision that we perceive changes in light / dark. However, things are not as simple as it seems.

In experimental work with rats, it is still possible to observe circadian responses to light in individuals who are congenitally blind (through genetic manipulation), thus suggesting that the information that regulates the endogenous circadian timing system is independent of the perception of the visual processes of the retina (Foster, 1993). Additionally, in mice that do not have cones and rods (congenital, through genetic manipulation), it is possible to observe that the circadian expression through light / dark is not significantly affected (Yoshimura and Ebihara, 1996, 1998).

From these facts, it is possible to assume that circadian synchronization does not depend on the visual perception of light. In the early 2000s, the existence of photoreceptor ganglion cells in the retina (ipRGCs) was discovered, with the presence of melanopsin. These photoreceptors have direct communication with suprachiasmatic nuclei, which play an essential role in the circadian time-keeping system (Beerson et al. 2002).

As in rodents, circadian system responses to light stimuli do not depend on the presence of retinal cones and rods. This role can be performed only with the ipRGCs, although they communicate indirectly with the cones and rods, receiving information that is also used to adjust circadian timing.

In recent studies, the authors describe how information processed by cones and ipRGCs has different roles in the circadian time-keeping system. Gooley et al. (2010), conducted a study with human beings based on a light exposure therapy in order to identify changes in melatonin secretion and changes in circadian expression. However, in addition to using a blue spectrum light (460 nm), which is the most sensitive wave frequency for ipRGCs, they also used a green spectrum light, which is more sensitive to the retinal photoreceptor cell system, the cones.

As a result, the authors observed that blue spectrum light has a more lasting role in suppressing melatonin when compared to green spectrum light, in which only an initial decrease in melatonin secretion is observed. However, the phase changes observed in the expression of the melatonin circadian rhythm are very similar, with green light therapy resulting in greater phase changes than blue light therapy.

Based on these data, the concern arises to prevent not only the emission of predominantly blue spectrum light by electronic devices at home. The green spectrum light can also generate phase delays in circadian rhythmicity, and as a consequence, delays in sleep and waking up times.

These results are extremely important when it comes to strategies for the development and optimization of light therapies in the treatment of sleep disorders and circadian rhythmicity. In addition to the time that light therapy is used, it is also necessary to manipulate the spectrum, duration and light patterns to stimulate the receptor systems of both the melanopsin and cones. It is also beneficial to choose the best type of intervention, whether melatonin suppression is needed during treatment. The authors also describe that the joint activation of melanopsins and cones in different light spectra can be effective in maintaining circadian expression in confined environments, such as space and submarine expeditions, among others. And we can also extrapolate the implementation of related therapies to situations where people are not routinely exposed to light, such as in long-stay institutions (nursing homes), hospitals (ICUs, for example) and even in strategies on long-term transcontinental flights, in which the time zone change generates rhythm disturbances caused by the social jet lag.

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