In this review, we propose that exposure to light at night alters metabolic function through disruption of the circadian system. The increase in exposure to light at night parallels the global increase in the prevalence of obesity and metabolic disorders. Since the advent of electric lights around the turn of the 20th century, exposure to artificial and irregular light schedules has become commonplace. 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. Moreover, misalignment between behavioral and molecular circadian clocks can result in obesity in both rodents and humans. Mice harboring mutations in circadian clock genes alter feeding behavior, endocrine signaling, and dietary fat absorption. 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. Appropriate circadian regulation is important for many homeostatic functions including energy regulation. 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. Most organisms display endogenously produced ∼24-hour fluctuations in physiology and behavior, termed circadian rhythms.

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