![]() The SCN is a bilateral mid-nucleus in the anterior ventral hypothalamus with approximately 10,000 neurons per nuclei. Then, two studies revealed that the ablation of this retino-recipient area in the ventral hypothalamus, harboring the suprachismatic nucleus (SCN), eliminates behavioral and hormonal rhythms in rats these studies gave the first recognition to the SCN as the central circadian clock in mammals. Interestingly, this study had already indicated that the RHT arises from retinal ganglion cells, which years later were identified as melanopsin-containing cells. Hence, in rats, the retino-hypothalamic tract (RHT) was characterized as the principal monosynaptic projection from the retina to the ventral hypothalamus. Therefore, one of the first experimental approach was to track pathways and brain targets using tracers injected into the eye. To recognize the endogenous structure able to tick every function in our body and to synchronize to day-night cycles, the first studies identified the anatomical pathway by which light enters to the body and entrains circadian rhythms. The Core of the Body Clock Entrained by Light: Following the Right Path The study of the circadian and non-circadian mechanisms underlying the effects of light on physiology is essential for the understanding of when and how light is beneficial or detrimental for health.Ģ. Furthermore, since retina projections target directly to other brain regions, beyond the circadian clock, light (artificial or natural) may also influence and regulate functions such as mood, cognition, and metabolism. Notwithstanding, because MEL is rhythmically secreted at night under the control of the body clock, light also affects the circadian system. One of principal effects of artificial light at night on physiology is the suppression of the hormone melatonin (MEL) this indicates that light regulates the neuroendocrine system. Today, it is well known that artificial light affects the physiology of living organisms, and mainly the biological functions with rhythmic properties. It is very likely that at that time people did not visualize what would be the impact of this event in biology and health. Edison invented the electric light bulb that was immediately used for both domestic and industrial tasks. In the last part of the industrial revolution (1879), Thomas A. Then, with the industrial revolution, a relevant event in the human history, several changes arrived to “facilitate” human life among them the discovery of electric light. In the beginning, organisms (including humans) regulated their daily activities according to the natural dawn and dusk. Therefore, sunlight impacts daily and seasonal physiology. Moreover, light effects on physiology are modulated by seasons, when the length and intensity of natural light change. Thus, sunlight is the most important natural signal to entrain our rhythms of sleep, feeding, body temperature, and metabolism. Knowing how the brain reacts to artificial light exposure, using diurnal rodent models, is fundamental for the development of new strategies in human health based in circadian biology.Įarth’s 24 h rotation induces cycles of day and night that synchronize the physiology of living organisms. The present review provides an overview of the effects of light at nighttime on physiology and behavior in diurnal mammals, including humans. ![]() Nevertheless, because of the diurnal nature of human physiology, it is also important to find and propose diurnal animal models for the study of the light effects in circadian biology. ![]() To decipher the brain mechanisms underlying these alterations, fundamental research has been conducted using animal models, principally of nocturnal nature (e.g., mice). The chronic exposure to light at nighttime has been correlated to mood alterations, metabolic dysfunctions, and poor cognition. Although electric light at night allows us “to live in darkness”, our current lifestyle facilitates nighttime exposure to light by the use, or abuse, of electronic devices (e.g., smartphones). While in the daytime, indoor artificial light is of lower intensity than natural sunlight, leading to a weak entrainment signal for our internal biological clock, at night the exposure to artificial light perturbs the body clock and sleep. However, in current times, natural sunlight has been replaced by artificial light in both day and nighttime. Natural sunlight permits organisms to synchronize their physiology to the external world. ![]()
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