Mammalian circadian systems consist of a hierarchy of oscillators at the cellular, tissue, and systems levels. A common molecular mechanism underlies the cell-autonomous circadian oscillator, yet this clock system is adapted to different functional contexts. The central suprachiasmatic nucleus (SCN) acts as a master pacemaker, driving rhythms in activity, feeding, body temperature, and hormones. Coupling within the SCN network ensures robustness and stability of the circadian system. Cell-autonomous clocks are embedded in metabolic pathways, highlighting their role in orchestrating metabolism. The SCN receives photic input from intrinsically photoreceptive retinal ganglion cells (ipRGCs), which express melanopsin and are sensitive to light. These cells, along with rod and cone photoreceptors, contribute to the SCN's function. The SCN contains approximately 20,000 neurons, each with a cell-autonomous circadian oscillator. These neurons are coupled, leading to coherent oscillations and precise circadian rhythms. The heterogeneity in intrinsic periods of SCN neurons allows for phase lability and plasticity, enabling adaptation to different photoperiods. Peripheral oscillators, found in various tissues, also contain cell-autonomous clocks. These clocks are influenced by the SCN and can be entrained by environmental cues such as temperature and feeding. The SCN controls peripheral oscillators through autonomic pathways, influencing hormone secretion, body temperature, and metabolic processes. The circadian system integrates signals from the SCN, peripheral clocks, and metabolic pathways, ensuring coordination of physiological functions. The circadian clock is regulated by a molecular feedback loop involving genes such as Clock, Bmal1, Per, and Cry. Additional feedback loops, including those involving Rev-erba and Rora, modulate clock function. The SCN is resistant to temperature entrainment, while peripheral oscillators are highly sensitive. The circadian system is also influenced by hormonal signals, such as glucocorticoids, and metabolic pathways, including the role of nuclear receptors and AMPK. Food and drug-sensitive oscillators, such as the food-entrainable oscillator (FEO) and methamphetamine-sensitive circadian oscillator (MASCO), can drive circadian rhythms independently of the SCN. These oscillators highlight the complexity of the circadian system and its integration with metabolic and physiological processes. The circadian system is a highly interconnected network, with feedback loops at multiple levels, ensuring the coordination of biological rhythms and the maintenance of homeostasis.Mammalian circadian systems consist of a hierarchy of oscillators at the cellular, tissue, and systems levels. A common molecular mechanism underlies the cell-autonomous circadian oscillator, yet this clock system is adapted to different functional contexts. The central suprachiasmatic nucleus (SCN) acts as a master pacemaker, driving rhythms in activity, feeding, body temperature, and hormones. Coupling within the SCN network ensures robustness and stability of the circadian system. Cell-autonomous clocks are embedded in metabolic pathways, highlighting their role in orchestrating metabolism. The SCN receives photic input from intrinsically photoreceptive retinal ganglion cells (ipRGCs), which express melanopsin and are sensitive to light. These cells, along with rod and cone photoreceptors, contribute to the SCN's function. The SCN contains approximately 20,000 neurons, each with a cell-autonomous circadian oscillator. These neurons are coupled, leading to coherent oscillations and precise circadian rhythms. The heterogeneity in intrinsic periods of SCN neurons allows for phase lability and plasticity, enabling adaptation to different photoperiods. Peripheral oscillators, found in various tissues, also contain cell-autonomous clocks. These clocks are influenced by the SCN and can be entrained by environmental cues such as temperature and feeding. The SCN controls peripheral oscillators through autonomic pathways, influencing hormone secretion, body temperature, and metabolic processes. The circadian system integrates signals from the SCN, peripheral clocks, and metabolic pathways, ensuring coordination of physiological functions. The circadian clock is regulated by a molecular feedback loop involving genes such as Clock, Bmal1, Per, and Cry. Additional feedback loops, including those involving Rev-erba and Rora, modulate clock function. The SCN is resistant to temperature entrainment, while peripheral oscillators are highly sensitive. The circadian system is also influenced by hormonal signals, such as glucocorticoids, and metabolic pathways, including the role of nuclear receptors and AMPK. Food and drug-sensitive oscillators, such as the food-entrainable oscillator (FEO) and methamphetamine-sensitive circadian oscillator (MASCO), can drive circadian rhythms independently of the SCN. These oscillators highlight the complexity of the circadian system and its integration with metabolic and physiological processes. The circadian system is a highly interconnected network, with feedback loops at multiple levels, ensuring the coordination of biological rhythms and the maintenance of homeostasis.