Circadian rhythms are endogenously produced biological rhythms with a period of approximately 24 hours that repeat every 24 hours. The majority of our physiological and behavioral activities are coordinated by the circadian system, including our sleep-wake cycle, body temperature and many endocrine and immune variables. The circadian system ensures our daily activities are both coordinated to one another as well as to the external environment. The circadian system is coordinated by the central biological clock located in the suprachiasmatic nucleus, or SCN, and located in the anterior hypothalamus. The SCN is entrained to Earth's 24 hour day via exposure to the light-dark cycle. Light is the strongest time cue marker (zeitgeber) that entrains and phase shifts the circadian system.

Circadian disruption occurs when our daily activities occur out of phase with daylight hours and commonly occurs in shiftworkers, international travelers and in psychiatric disorders, such as depression and bipolar disorder. The effects of circadian disruption can include disturbed sleep-wake behavior, changes in appetite and metabolism, and changes in alertness and neurocognitive function. Circadian re-entrainment can be achieved via a number of mechanisms, including appropriate timing of bright light, or blue light, administration of melatonin and some instances forced changes in sleep-wake timing.