Circadian system is master clock of daily sleep-wake cycle. It is able to create periodic activity with a period of almost 24 hours autonomously, and is able to entrain itself to day-night cycle of environment using with daily change of sunlight as time clue. Circadian rhythm sleep disorders (CRSDs) are defined as sleep disorders caused by misalignment between internal circadian clock and social schedule. Insomnia, hypersomnia, headache or intestinal symptoms are common complaints of CRSD patients, and these symptoms may deteriorate patients' social function. Jet lag disorder and shift work disorder are originated by acute shift of social schedule that exceed ability of circadian entrainment. However, pathophysiology of other CRSDs are not fully cleared. Treatments of CRSDs aim to facilitate circadian entrainment to desirable schedule with time clues at proper timing of circadian system. Bright light therapy (BLT) and melatonin administration are effective. Hypnotic administration is not effective in most cases.