The human clock challenged by urban life

Abstract

The biological clock has evolved to adapt the functioning of organisms to a predictable light–dark cycling environment, but the human clock copes with urban environments in which the day–night natural cycle has dramatically altered. Novel research presented in this symposium will show the modulations of the ancestral clock on human behavior, performance, and health in people living in heavily intervened urban environments that differ greatly from the original human natural environment. Objectives: Although the hypothalamic suprachiasmatic nuclei (mammalian master circadian clock) can set the timing of different activities without any cyclic input from the environment, the light-dark cycle is key for resetting the circadian system to the external 24-h day. Other cues can also act as circadian entrainers such as food intake, programmed exercise, and stress. The precise mechanisms by which the clock is modulated by these ambient and social cues have been mostly studied in controlled laboratory conditions in animal model systems. Urban environments, in which most humans live, constitute a challenging world for the circadian clock because the light-dark cycle has become less contrasted and because social pressures, instead of natural ones, govern school, work, and entertainment schedules. Self-reported data together with objective actimetry, light exposure, and hormone measurements, have recently given insight on how the human clock is still controlled by its ancestral entrainers and have shown that circadian misalignment has consequences on behavior, performance, and health. This symposium will contribute a more comprehensive view of the adaptations of the biological clock to daily human life when challenged by environmental (light), physiological (exercise), social interactions (mother-infant dyad), and social pressures (work shifts) in health and disease. All these perspectives will come together to show that timing is crucial for keeping the human brain in good shape.