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Circadian rhythmicity is an important component of physiological processes which provides them with a 24-hour temporal organization and adjustment to cyclical changes in the environment. Circadian rhythms are controlled by a network of endogenous clocks, comprising the main clock in the suprachiasmatic nuclei of the hypothalamus and many secondary clocks in the brain and peripheral tissues. All aspects of energy metabolism, from food intake to intracellular signaling pathways, are strongly influenced by circadian rhythmicity. In turn, meal timing is an efficient synchronizer (time-giver) to set the phase of the peripheral clocks, while the suprachiasmatic clock is synchronized by ambient light. In certain nutritional conditions (i.e., low- or high-calory diets), metabolic factors remaining to be identified modulate the functioning of the suprachiasmatic clock. Animal models of obesity and diabetes show circadian alterations. Conversely, when circadian rhythmicity is disturbed, either due to genetically defective circadian clocks, or to circadian desynchronization (chronic light exposure or repeated meals at odd times of the cycle), lipid and glucose metabolism is deregulated. The metabolic impact of circadian desynchronization justifies the development of preventive or therapeutic strategies that could rely, among others, on dietary interventions combining timed meals and specific composition., (© Société de Biologie, 2015.)

Cellular rhythms represent a field of choice for studies in system biology. The examples of circadian rhythms and of the cell cycle show how the experimental and modeling approaches contribute to clarify the conditions in which periodic behavior spontaneously arises in regulatory networks at the cellular level. Circadian rhythms originate from intertwined positive and negative feedback loops controlling the expression of several clock genes. Models can be used to address the dynamical bases of physiological disorders related to dysfunctions of the mammalian circadian clock. The cell cycle is driven by a network of cyclin-dependent kinases (Cdks). Modeled in the form of four modules coupled through multiple regulatory interactions, the Cdk network operates in an oscillatory manner in the presence of sufficient amounts of growth factor. For circadian rhythms and the cell cycle, as for other recently observed cellular rhythms, periodic behavior represents an emergent property of biological systems related to their regulatory structure.