Circadian rhythm sleep disorders are caused by a mismatch between the sleep - wake pattern and circadian phase. Sleep at the circadian high is impaired, while alertness is reduced during wakefulness. This issue discusses biological timekeeping, consequences of circadian disruption in animal models, influence of circadian system on disease severity, effect of light on circadian physiology, melatonin, non-24-hour disorder, irregular sleep-wake disorder, advanced sleep phase, delayed sleep phase, jet lag, shift work, circadian disruption and psychiatric disorders, and winter depression.