Circadian Rhythms: Light and Mood

The circadian system is made up of different types of molecular clocks that track behavioral output under hours of the day, typically running under a full 24 hours. In an article about how this system controls feeding behavior in flies gave an interesting insight: that central and peripheral clocks both contribute to feeding, while under the fat body clock, there was no relation to feeding systems; instead, it was suggested that it may rely under light cues (Fulgham et al., 2021).

Although flies may seem very different from humans, there is no denying the reasons we use them as a model organism, since they are readily available for research. Due to similarities in circadian rhythm displays, ideas about how they relate to us can come about. Perhaps the idea that a molecular clock in flies may have its behavior affected due to light can correspond to the idea that humans, too, have our external behaviors affected by light.

In humans, previous research believed that only rods and cones responded to light as it went through the retina in the eye, but a third photoreceptor called intrinsically photosensitive retinal ganglia cells, or iPRGCs, has also been found to respond to light, despite not being part of the rod-cone system (Zuckerman, 2020). These photoreceptors have been found to target the suprachiasmatic nucleus, or SCN, which regulates and deals with the peripheral clocks of the circadian system; the effects light has on learning and mood relates to the idea that the SCN, under these photoreceptors, has its own independent function (Fernandez et al., 2018). Research has also shown that the iPRGCs have their own circuitry in the perihabenular nucleus of the thalamus, a region that connects to regulating mood (Fernandez et al., 2018).

When it comes to mood, specifically thinking about SAD, or seasonal affective disorder, a type of depression that individuals may experience when there is less natural light in their environment due to the fall and winter seasons, and how light can affect this disorder through iPRGCs, which are connected to the circadian system due to the idea that they target the SCN, can begin the discussion of the possible connection between circadian rhythms and mood. Light changes in the environment have been shown to affect mood; mice studies explain that there have been mood changes in the animals due to light disruption in the environment, and these changes usually take a bit of time, around two weeks, to occur (Zuckerman, 2020). This may help reveal why individuals with SAD have a hard time recognizing their changing mood; it is because it takes time for light to truly affect them (Zuckerman, 2020).

In terms of natural and artificial sunlight, the sun itself and going outside can help balance circadian rhythms, while bright artificial light, from electronics, for example, may hinder that balance (Zuckerman, 2020). The appeal of the affect different types of light might have on improving or impairing circadian rhythms and disorders such as SAD is one that must be investigated. In a world advancing in technology and global warming, would artificial or natural light contribute to a positive or negative outcome in individuals’ overall mood or individuals with disorders such as SAD? How would iPRGCs, which fall under the peripheral clock system, be affected by the different types of light, and would this research be beneficial to society in the future? These are only a few hypothetical questions, but in a world that is vastly changing, the answers may not be far behind.

References

Fernandez, D. C., Fogerson, P. M., Ospri, L. L, Thomsen, M. B., Layne, R. M., Severin, D., Zhan, J., Singer, J. H, Kirkwood, A., Zhao, H., Berson, D. M., & Hattar, S. (2018). Light affects mood and learning through distinct retina-brain pathways. Cell, 175(1), 71-84.

https://doi.org/10.1016/j.cell.2018.08.004

Fulgham, C. V., Dreyer, A. P., Nasseri, A., Miller, A. N., Love, J., Martin, M. M., Jabr, D. A., Saurabh, S., & Cavanaugh, D. J. (2021). Central and Peripheral Clock Control of Circadian Feeding Rhythms. Journal of Biological Rhythms, 36(6), 548-566. https://doi:10.1177/07487304211045835

Zuckerman, H. (2020). How does sunlight affect our mood? https://www.brainfacts.org/diseases-and-disorders/mental-health/2020/how-does-sunlight-affect-our-mood-120720