Figure 2.

Impact of modern environments on evolutionarily programmed circadian functions. Sunlight, temperature, physical activity, and food intake serve as basic entraining cues, or zeitgebers, that coordinate tissue-specific circadian processes to cumulatively define whole organismal physiology. Among these zeitgebers, light is the chief synchronization cue and acts to reset the master clock (A) in the hypothalamic SCN each day, which then relays signaling to peripheral tissues. Evolutionarily fine-tuned, tissue-specific circadian processes discussed in this review are depicted, including: B, heat production by brown adipose; C, energy storage by white adipose; D, fuel source management between carbohydrate and lipid substrates in the liver; E, distribution or circulation of blood-borne factors, hormones, and metabolites by the heart; F, control of blood glucose levels by the pancreas; G, capacity for movement and activity by skeletal muscle; and H, food processing and nutrient extraction by the gut microbiome. A combination of light pollution from artificial light sources, sedentary lifestyles largely lacking physical activity, continuous access to high-calorie foods, and living conditions maintained at constant ambient temperature have all contributed to the disruption of circadian fitness.