Not until the 19th century theories on sleep were based upon experimental findings in animal and humans. The so-called 'hypnotoxin theory' culminated, when Legendre and Piéron successfully induced sleep in a dog by transmission of cerebrospinal fluid from a dog deprived of sleep. The main discussion concerning the origin of sleep has been the question if sleep is a passive or an active state. Similarities with coma, the positive Babinski sign and pathoanatomical findings in patients who died after encephalitis lethargica were the arguments for the 'deafferentiation hypothesis'. Bremer's classical brainstem-transsections in cats confirmed this idea. Pavlov was the major representative of the idea that sleep was due to a general inhibition of the brain. Hess induced physiological sleep in cats by electrical stimulation of the diencephalon, proving the active nature of sleep. The introduction of the EEG in animals by Caton and in humans by Berger allowed for the first time the measurement of sleep depth without waking the sleeper. After discovery of the REM sleep periods by Aserinsky and Kleitman in 1953 and the demonstration of periodical sleep cycles by Dement and Kleitman, polysomnography with simultaneous whole night recording of EEG, EMG, electrooculogram and other physiological parameters was established as the major diagnostic tool in sleep disorders. One of the most important questions about the function of sleep is still unresolved. NREM sleep is believed to have a restorative function, whereas REM sleep might be involved in learning processes. According to the sleep interpretation of Sigmund Freud, the dream content represents endogenous wishes which cannot be expressed during wakefulness because of an internal 'sensor'. A more recent theory by Hobson explains the dreams by a very unspecific brainstem activity occurring during REM sleep which projects to the frontal brain and activates stored memory. The most important sleep disease of the 20th century is certainly the sleep-apnea syndrome. Charles Dickens described in his 'Pickwick Papers' subjects with this illness already 150 years ago. The pathogenetic significance of the apneas during sleep, however, were recognized in 1965 only by Gastaut and at the same time by Jung and Kuhlo. Treatment for insomniacs was restricted for many years to alcohol, opium and barbiturates. Following the horrible sequelae of thalidomide therapy in 1956, a more efficient treatment was available through the introduction of benzodiazepines after 1960.