The Effects of Light at Night on Circadian Clocks and Metabolism

Abstract

Most organisms display endogenously produced ∼ 24-hour fluctuations in physiology and behavior, termed circadian rhythms. Circadian rhythms are driven by a transcriptional-translational feedback loop that is hierarchically expressed throughout the brain and body, with the suprachiasmatic nucleus of the hypothalamus serving as the master circadian oscillator at the top of the hierarchy. Appropriate circadian regulation is important for many homeostatic functions including energy regulation. Multiple genes involved in nutrient metabolism display rhythmic oscillations, and metabolically related hormones such as glucagon, insulin, ghrelin, leptin, and corticosterone are released in a circadian fashion. Mice harboring mutations in circadian clock genes alter feeding behavior, endocrine signaling, and dietary fat absorption. Moreover, misalignment between behavioral and molecular circadian clocks can result in obesity in both rodents and humans. Importantly, circadian rhythms are most potently synchronized to the external environment by light information and exposure to light at night potentially disrupts circadian system function. Since the advent of electric lights around the turn of the 20th century, exposure to artificial and irregular light schedules has become commonplace. The increase in exposure to light at night parallels the global increase in the prevalence of obesity and metabolic disorders. In this review, we propose that exposure to light at night alters metabolic function through disruption of the circadian system. We first provide an introduction to the circadian system, with a specific emphasis on the effects of light on circadian rhythms. Next we address interactions between the circadian system and metabolism. Finally, we review current experimental and epidemiological work directly associating exposure to light at night and metabolism.

The role of melatonin in diabetes: therapeutic implications

“…Likewise other physiological functions, glucose metabolism is regulated by circadian system (43,44). In an experiment, study Clock mutant mice showed lack of rhythmicity in the action of insulin, a condition which was reversible once the clock protein was reintroduced (43).…”

“…Melatonin inhibits glucose mediated release of insulin from pancreatic cells emphasizing its activity in the function of insulin (72). Suppression of melatonin secretion by nocturnal light exposure could be a critical factor for type 2 diabetes development (44). Furthermore, MT1 receptors are involved in the modulation of glucose homeostasis in mice and might stimulate insulin to induce glucose uptake (73).…”

The effects of light pollution on biological rhythms of birds: an integrated, mechanistic perspective

“…The molecular clockwork is well described in mammal species, and so is the integration of non visual photic information by photoreceptors in the inner retina. Such detailed description of the mammalian circadian system has helped researchers interested in the effects of light pollution to predict the pathways through which light at night may affect circadian rhythms in mammals (Fonken and Nelson 2014 ). For instance, as the expression of the negative clock gene Per is normally low at night, light administered during the subjective night should significantly alter the pattern of Per expression and consequently affect other components of the circadian clock.…”

“…For instance, as the expression of the negative clock gene Per is normally low at night, light administered during the subjective night should significantly alter the pattern of Per expression and consequently affect other components of the circadian clock. Dim light at night (~5 lux) has been shown to alter circadian clock gene expression of mice (Fonken et al 2013a ; Fonken andNelson 2014 ) and Siberian hamsters (Bedrosian et al 2013 ), including Bmal1, Per1, Per2, Cry1, Cry2, and Rev Erb. These effects have been reported in both the SCN and the liver, with the latter associated with increased body weight independently of daily caloric intake (Fonken et al 2013a ).…”

Electric light, particularly at night, disrupts human circadian rhythmicity: is that a problem?

“…There is a large and growing literature on the effects of circadian gene knockouts (KO) in mice, and polymorphisms in humans, on disease risk [57][58][59][60][61][62][63]. These studies suggest the possibility that absent or altered function of circadian genes may increase the risk of some diseases in people.…”