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.

Keywords: SCN; cardiovascular; circadian rhythms; hormonal rhythms; jet lag; light; melatonin; metabolism; sleep.

Figure 1

The circadian clock in mammals. In SCN neurons at the start of the circadian day, BMAL1 and CLOCK form a heterodimer (1) that binds to E-box sequences in the promoters of the Cry and Per genes (2) to activate their transcription (3). This marks the beginning of the circadian day. The gene products of Per and Cry accumulate in the cytoplasm, dimerize (4), and then form a complex that translocates into the nucleus (5) to interact with CLOCK and BMAL1, ultimately repressing their own transcription (6). This process takes approximately 24 h. LAN affects the timing of this transcription/translation cycle leading to temporal misalignments affecting physiology and behavior. CCGs = circadian clock genes. Figure was created using Biorender.com (accessed on 13 January 2022).