The circadian system in humans encompasses all organs, tissues and cells. Coordination of central and peripheral clocks and synchronization of cellular clocks within the brain regulate daily phases, neurophysiology and behavior. The mechanism of action of the circadian system is complex, but is centered on the paired structure of the suprachiasmatic nuclei (SCN) that serves as a pacemaker in humans. Light adjusts the phase of the SCN oscillator to the environmental light-dark cycle. Light therapy has been developed for clinical use and many apparatus and parameters have been extensively studied. Bright light therapy is the treatment of choice for seasonal affective disorder and circadian rhythm sleep disorders. Cumulative studies support the efficacy of light therapy for some clinical conditions which are characterized by seasonality or disrupted circadian rhythms. The benefit of light therapy is significant and warrants further clinical studies to optimize the treatment effect.