This chapter provides an overview of idiopathic hypersomnia together with a discussion on the nosological limits of the condition. It presents the history, clinical features, diagnostic procedures, treatment and differential diagnosis of idiopathic hypersomnia. All efforts to clarify demographics, pathophysiology and treatment have been hampered by the nosological uncertainty on the borders of idiopathic hypersomnia. The latest International Classification of Sleep Disorders confirmed the distinction of two forms of idiopathic hypersomnia and used a new wording of idiopathic hypersomnia with long sleep time and idiopathic hypersomnia without long sleep time. The diagnosis of idiopathic hypersomnia with long sleep time is mainly based on clinical features and on the absence of associated symptoms such as snoring or cataplexy. However, polysomnography and the multiple sleep latency test (MSLT) are indispensable to rule out other conditions. Further research is necessary to specify the limits of idiopathic hypersomnia with and without long sleep time.

This chapter discusses the neurochemistry of excessive daytime sleepiness (EDS) with various etiologies. It presents a brief discussion of basic sleep physiology and narcolepsy symptoms to explain the specific neurochemistry of hypersomnia. The significant roles hypocretin deficiency and postnatal cell death of hypocretin neurons as the major pathophysiological process underlying narcolepsy with cataplexy emerged from a decade of investigation, employing both animal and human models. The chapter talks about idiopathic hypersomnia and hypocretin non-deficient primary hypersomnia. It explains how hypocretin ligand deficiency may cause narcolepsy phenotype. Narcolepsy symptoms can also occur during the course of other neurological conditions, and discovery of hypocretin ligand deficiency in idiopathic narcolepsy has led to new insights into the pathophysiology of symptomatic narcolepsy and EDS. The metabolic data may support the hypothesis of a primary deficient arousal system in patients with idiopathic hypersomnia.

This chapter discusses the case of a 56-year-old man who presented to the sleep clinic complaining of frequent episodes of feeling paralyzed as he was going to sleep. It presents the clinical history, examination, follow-up, treatment, diagnosis, and the results of the procedures performed on the patient. Examination revealed a pleasant, thin, middle-aged man, in no acute distress, who looked younger than his stated age. Based on the history, the diagnosis of isolated or familial sleep paralysis was made and the patient was started on clomipramine 25 mg at bedtime. Differential diagnosis includes narcolepsy if excessive daytime sleepiness (EDS) is present or a history of hypnic hallucinations and cataplexy can be elicited. Although daytime sleepiness is necessary to make the diagnosis of narcolepsy, sleep paralysis can also occur in other conditions that present with daytime sleepiness such as sleep deprivation, sleep-related breathing disorder and idiopathic hypersomnia.

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.