Excessive somnolence can be a consequence of lifestyle, environmental and circadian influences, medical disorders, drugs or substances. Disorders of excessive somnolence include insufficient sleep, narcolepsy, idiopathic hypersomnia, recurrent hypersomnia and hypersomnia associated with neurological disorders, psychiatric disorders and internal disorders. This chapter focuses on hypersomnias of central origin and arising from insufficient sleep. It describes epidemiology, pathophysiology, diagnosis, and therapeutic options for each of the excessive somnolence disorder. Hypersomnia may be caused by an underlying medical or neurological disease. Secondary hypersomnias described in the chapter include Parkinson's disease, genetic disorders, endocrine disorders and psychiatric disorders. Multiple chemical substances, especially organic solvents with chronic exposure, may cause hypersomnia. Pancreatic, adrenal or renal insufficiency, and hepatic encephalopathy are other causes. Hypersomnia may occur as a side effect of many drugs, due either to the soporific effects of the drug or to the effect of the drug on reducing night-time sleep.