Abstract
Natural sunlight permits organisms to synchronize their physiology to the external world. However, in current times, natural sunlight has been replaced by artificial light in both day and nighttime. 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. 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). The chronic exposure to light at nighttime has been correlated to mood alterations, metabolic dysfunctions, and poor cognition. To decipher the brain mechanisms underlying these alterations, fundamental research has been conducted using animal models, principally of nocturnal nature (e.g., mice). 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. The present review provides an overview of the effects of light at nighttime on physiology and behavior in diurnal mammals, including humans. 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.
Highlights
Earth’s 24 h rotation induces cycles of day and night that synchronize the physiology of living organisms [1]
Because behavioral, cellular, and molecular responses to light are similar between humans and diurnal rodents, but opposite to nocturnal rodents, it would be interesting and relevant to propose the use of diurnal animal models to study the mechanisms underlying the effects of light on human physiology (Figure 1)
Since light at night impacts extra-suprachismatic nucleus (SCN) brain regions differentially between nocturnal vs. diurnal rodents [49], and some of them are importantly implicated in the regulation of mood (LHb) [102], research on the mechanisms underlying the effects of nighttime light exposure (NLE) on moodrelated behavior in diurnal species remains to be explored
Summary
Earth’s 24 h rotation induces cycles of day and night that synchronize the physiology of living organisms [1]. Light effects on physiology are modulated by seasons, when the length and intensity of natural light change. It is well known that artificial light affects the physiology of living organisms, and mainly the biological functions with rhythmic properties. 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 [3]. Notwithstanding, because MEL is rhythmically secreted at night under the control of the body clock, light affects the circadian system. Since retina projections target directly to other brain regions, beyond the circadian clock, light (artificial or natural) may influence and regulate functions such as mood, cognition, and metabolism [4]. 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
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