The circadian clock is present in a diverse group of organisms that helps to integrate changes in the external environment and internal physiology. It allows optimal timing of behavioral, physiological, and molecular processes with respect to the surrounding environment. Light (or photic cue) is the most potent environmental cue that synchronizes the circadian rhythms in animals. In mammals, photic cues are perceived by retinal photoreceptors, which through the retinohypothalamic tract are communicated to the central pacemaker, the suprachiasmatic nucleus (SCN). Efferent projections from the SCN innervate the pineal gland and regulate the production and secretion of melatonin. At the cellular level, a set of clock genes comprise the transcriptional translational feedback loop (TTFL) with a periodicity of approximately 24 h. Disruption of the clock gene functions within the SCN can lead to various circadian rhythm disorders. The circadian disruptions include altered sleep-wake cycle, misaligned feeding time, and misalignment of central and peripheral rhythms. Circadian misalignments are also associated with neurological, psychiatric, and metabolic disorders. The symptoms of many of these disorders can be treated using the principles of chronotherapy, which optimizes the timing of medical treatment according to the internal circadian rhythms. Modifications of the sleep-wake cycle through bright light therapy and melatonin treatment can alleviate the symptoms of circadian rhythm sleep disorders. The implementation of chronotherapy will maximize the therapeutic outcomes of drug treatment while minimizing the harmful side effects.