Insomnia is the most common sleep complaint. Insomnia is not a disease itself but mostly a clinical sign of an underlying disease. Degenerative and vascular diseases involving the central nervous system (CNS) may impair sleep either as a result of the brain lesion or because of illness-related discomfort (motor immobility, social and familial impairment, depression, drugs). Some neurological conditions characterized by movement disorders that start or persist during sleep hinder sleep onset and/or sleep continuity, causing a poor sleep complaint. CNS lesions and/or dysfunction in three specific neurological conditions (fatal familial insomnia, Morvan's chorea, and delirium tremens) impair the basic mechanisms of sleep generation inducing a syndrome in which the inability to sleep is consistently associated with motor and sympathergic overactivation. Agrypnia excitata is the term that aptly defines this generalized overactivation syndrome.