Secondary disorders of awakening should be distinguished from primary disorders, narcolepsy, idiopathic hypersomnia, recurrent hypersomnia, the causes of which are still unknown despite regular progress in the knowledge of the pathophysiology of narcolepsy. By definition secondary disorders of awakening are due to clearly identified causes of various origins. Two main types of secondary disorders of awakening have been distinguished: those depending on more or less voluntary sleep curtailment or on psychotropic or non psychotropic medications and those consecutive to different disorders, respiratory, neurologic, traumatic, psychotropic, infectious, metabolic, endocrinologic, and insomnia. Some of these disorders, frequent or very frequent, are polysomnographically investigated, night and day, enabling to assess in each case the type and severity of sleepiness. Others are only clinically evaluated. Disorders of awakening secondary to neurologic conditions and to a lesser extent to infectious conditions offer a special opportunity to study the anatomical basis of these disorders. They are granted more space.