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Up to the beginning of modern sleep research, theories about sleep have dominated the scene. With their philosophic background, they must be considered as erroneous by present standards. The history of sleep-inducing drugs has followed its own pathways, which were independent from sleep research. Up to the 19th century, opium and alcohol were used predominantly as hypnotics, later, empirically found chemical compounds were added. Today, new drugs are tested by methods of modern sleep research before they reach the market. Electrophysiology, whose origins are described, formed the basis of electroencephalography (EEG). The history of EEG is an important part of the present exposé. The discovery of rapid eye movements (REM) during sleep has been one of the most important achievements in modern sleep research. It led to a new stage classification - which is still used today - as well as to the discovery of sleep cycles. Subsequently, polysomnography has been increasingly used. Additional methods are actometry and the spectral analysis of the sleep EEG. Research of endogenous sleep substances such as "Delta Sleep Inducing Peptide (DSIP) has been actively pursued in the last 25 years. It is unlikely that one particular endogenous substance underlies sleep regulation. Rather a complex system involving different neurotransmitters must be postulated. Sleep disorders medicine is a new medical discipline which has undergone a rapid development. In the USA more than 1000 "sleep disorders centers" have arisen in the past few years. A description of the new 1990 classification of sleep disorders is provided, and narcolepsy, sleep apnea syndrome and some disturbances of the sleep-wake cycle are briefly characterized.