Circadian Rhythms Disrupted by Light at Night and Mistimed Food Intake Alter Hormonal Rhythms and Metabolism

Abstract

Availability of artificial light and light-emitting devices have altered human temporal life, allowing 24-hour healthcare, commerce and production, and expanding social life around the clock. However, physiology and behavior that evolved in the context of 24 h solar days are frequently perturbed by exposure to artificial light at night. This is particularly salient in the context of circadian rhythms, the result of endogenous biological clocks with a rhythm of ~24 h. Circadian rhythms govern the temporal features of physiology and behavior, and are set to precisely 24 h primarily by exposure to light during the solar day, though other factors, such as the timing of meals, can also affect circadian rhythms. Circadian rhythms are significantly affected by night shift work because of exposure to nocturnal light, electronic devices, and shifts in the timing of meals. Night shift workers are at increased risk for metabolic disorder, as well as several types of cancer. Others who are exposed to artificial light at night or late mealtimes also show disrupted circadian rhythms and increased metabolic and cardiac disorders. It is imperative to understand how disrupted circadian rhythms alter metabolic function to develop strategies to mitigate their negative effects. In this review, we provide an introduction to circadian rhythms, physiological regulation of homeostasis by the suprachiasmatic nucleus (SCN), and SCN-mediated hormones that display circadian rhythms, including melatonin and glucocorticoids. Next, we discuss circadian-gated physiological processes including sleep and food intake, followed by types of disrupted circadian rhythms and how modern lighting disrupts molecular clock rhythms. Lastly, we identify how disruptions to hormones and metabolism can increase susceptibility to metabolic syndrome and risk for cardiovascular diseases, and discuss various strategies to mitigate the harmful consequences associated with disrupted circadian rhythms on human health.

Dozing Off With Drosophila: The Effect of Disrupted Circadian Rhythms and Sleep Disturbance on Mortality, Mood, and Addiction

“…Mistimed light exposure can severely desynchronize biological and behavioral rhythms causing many negative health consequences. 41 Our results underscore the importance of intact circadian rhythms and sleep-wake cycles to physical and mental health. Future studies must evaluate genetic and physiologic mechanisms underlying poor outcomes associated with disrupting sleep and circadian rhythms, thereby allowing targeted interventions.…”

Disturbances of Hormonal Circadian Rhythms by Light Pollution

“…Melatonin is the best-studied hormone in the ALAN context in different systematic classes of animals [ 69 ] and has also been reviewed [ 70 ]. The nocturnal suppression of MEL is generally considered the best marker of light pollution because its concentrations can be rather easily measured in all biological fluids, not only in plasma and serum, but also noninvasively in saliva or urine.…”

Hormones—Modulators of metabolism

“…The suprachiasmatic nucleus is a cluster of nerve cells in the hypothalamus at the base of the brain and function as primary circadian clock in humans. 13 Most biological processes, like metabolism and hormone secretion, oscillate with a 24-h rhythmicity in correspondence to the cyclic environment. 14 The gene expression of the circadian clock system is acutely affected by hormones like insulin, leptin, ghrelin, and glucagon.…”

“…This coordination between the nervous and endocrine systems occurs through hormonal secretion. The suprachiasmatic nucleus is a cluster of nerve cells in the hypothalamus at the base of the brain and function as primary circadian clock in humans 13 . Most biological processes, like metabolism and hormone secretion, oscillate with a 24‐h rhythmicity in correspondence to the cyclic environment 14 .…”