In mammals, the master circadian clock that drives many biochemical, physiological and behavioral rhythms is located in the suprachiasmatic nuclei (SCN) of the hypothalamus. Generation and maintenance of circadian rhythms rely on complex interlaced feedback loops based on transcriptional and posttranscriptional events involving clock genes and kinases. This clock serves the purpose to organize an organism's biochemistry on a 24 h time scale thereby avoiding interference between biochemical pathways and optimizing performance. Synchronization to environmental 24 h oscillations tunes physiological processes optimally with nature. In this review, I briefly describe the principle of the clock mechanism, its synchronization to the environment and consequences on health when the circadian clock is disrupted.