The Circadian Dance. Impact of Night-Light on Your Brain

On our planet, day and night alternate daily. Throughout evolution, all living beings adjusted their functions to this alternation, giving rise to the circadian rhythms of all physiology. Artificial light at night (ALN) has brought benefits to society, but it is an emerging problem for both humans and nature [see UNAM International 3, “Dark Skies,” p. 108, and UNAM International 9, “A Window to the Universe from Baja California,” p. 102]. The ALN that originates from cities radiates luminosity to natural areas, reducing dark conditions and generating “light pollution” (Bará & Falchi, 2023). An example of the adverse effect of ALN is the opening of cactus flowers at night:  when exposed to ALN, they modify their flowering pattern, which in turn impacts the insects that pollinate them. ALN also affects other species at the reproduction, feeding, and migration levels (figure 1).

Figure1. Effects of artificial light at night (ALN)

In humans and laboratory animals, ALN is associated with sleep problems, increased oxidative stress, inflammation, metabolic alterations, heart problems, and reproductive alterations. It is an emerging problem that affects all living beings exposed to it..

Light reaches the brain through the eyes, forming the visual pathway that allows us to see shapes, colors, and images. Other areas of the brain also learn about the intensity of light through specialized cells that project to the biological clock, to hypothalamic nuclei that regulate reproduction and feeding, to the thalamus that regulates emotions, and to the pineal gland responsible for the secretion of melatonin (figure 2). With light and dark information, these brain areas adjust basic functions for survival and determine physical and mental health. We are studying how ALN affects these brain regions.

Figure 2. The path of light to the brain

Retinal ganglion cells transmit light information to the biological clock, to hypothalamic and thalamic nuclei that regulate basic functions such as reproduction, sleep, metabolism, learning, emotions, and the immune system. Therefore, the information of the light-dark cycle is essential for physical and mental health.

THE ALN ALTERS THESE CYCLES BY MODIFYING THE ESTIMATION OF NIGHT. THE TRAGIC CONSEQUENCE IS REDUCED REPRODUCTIVE SUCCESS

The biological clock signals to the whole body when it should increase or decrease the intensity of its activity and thus generates circadian rhythms that adapt functions to day and night. Light-dark information is the primary time guide; consequently, the ALN causes confusion in the biological clock, which loses its ability to coordinate the temporal order and determine the intensity of physiological and behavioral rhythms. Individuals enter a state of circadian disruption in which certain functions manifest themselves at different times and intensities than they should (Fishbein et al., 2021). As a result, there is sleep disturbance, hormonal imbalance, and physical exhaustion that favors the development of chronic diseases. 

Light information also reaches hypothalamic nuclei that regulate reproduction (figure 2). Many species breed during a specific season of the year; some on long days (spring/summer) and other species on short days (autumn/winter). This condition is important because it helps the young to be born when the climate and food availability are ideal for their survival. The ALN alters these cycles by modifying the estimation of night; the tragic consequence is reduced reproductive success. Although humans reproduce year-round, ALN induces shorter, more irregular menstrual cycles. In areas with higher light pollution, longer birth-labor duration, and lower neonatal weight have been reported (Moralia et al., 2022). Our studies indicate that in rodents, offspring born to mothers exposed to ALN during pregnancy have higher birth weight, neuroinflammation in adolescence, and behavioral alterations in adult life (figure 3).

Figure 3. Night-light alters rodents physiology

The image shows the main findings by or research group after exposing Wistar rats to a light-dark cycle (LO) or artificial light at night (LAN), producing alterations in several physiological parameters.

Light information reaches brain structures such as the lateral hypothalamus, which regulates hunger and satiety responses (figure 2), while the light signal in the supraoptic nucleus increases circulating glucose levels. Cortisol, a hormone that favors the accumulation of adiposity and inflammation, also increases with ALN. In our laboratory, it has been observed that exposure to ALN in rodents promotes increased adipose tissue deposits, weight, and circulating levels of glucose, triglycerides, and corticosterone (figure 3). In humans, there is little information on the impact of ALN on metabolism, but clinical observations indicate that it is a risk factor for developing obesity, metabolic syndrome, and type II diabetes. 

Exposure to ALN with an intensity equivalent to a computer screen is sufficient to inhibit the release of melatonin, a hormone produced in the pineal gland only in dark conditions. Melatonin has antioxidant and anti-inflammatory properties, making it important for cell repair and immune system response processes (Claustrat & Leston, 2015). The lack of melatonin by ALN alters all these functions. 

Our group demonstrated that rats exposed to ALN and an infectious agent have an exacerbated response in signs of illness, such as fever, weight loss, and appetite loss. We also observed that ALN-exposed rats inoculated with tumor cells developed larger volume tumors. These findings suggest that people who are constantly exposed to ALN, such as night workers, may be more likely to develop breast cancer (Wei et al., 2022). 

Because retinal ganglion cells project to the thalamus, where emotional responses are processed (figure 2), we show that rats exposed to ALN for eight weeks develop depressive and anxious behaviors (figure 3). 

CONCLUSION
The lack of darkness at night is an adverse condition for all modern society and for the species that live near large cities. Basic survival functions are affected. It is necessary to regulate, reduce, and control the use of light at night. Dim and warm light, with yellow or orange tones better than white or blue light, which resemble bright midday light. To sleep, it is recommended to avoid the filtration of external light with thick curtains or eye masks.

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