The document discusses the different types of circadian rhythm sleep disorders (CRSD). There are six types that fall into two categories - intrinsic or extrinsic. Intrinsic disorders involve neurological causes and include delayed sleep phase syndrome, advanced sleep phase syndrome, non-24-hour sleep-wake syndrome, and irregular sleep-wake disorder. Extrinsic disorders are due to social or environmental factors and include shift-work sleep disorder and jet lag. CRSDs occur when there is an abnormality in a person's natural sleep-wake cycle and can lead to psychological and functional difficulties if not properly diagnosed and treated.