The development of sleep research can be divided into two main periods. The first one was initiated in 1863 by the first systematic measurement of the depth of sleep, the second in 1953 by the discovery of recurrent episodes of rapid eye movements in sleep. The main methodological procedure in the first of these two periods was the measurement of a single physiological variable, while beginning with long-term measurements of the electroencephalogram (EEG) in sleep, multi-channel, polygraphic recording became the method of choice for sleep studies. Although rhythmic changes in the ultradian frequency range of one to 2 h were observed early in many variables during sleep (movements, autonomic functions, penile erections), the recognition of the existence of two different states of sleep (rapid eye movement (REM) and non-rapid eye movement (NREM sleep)) was contingent upon a ‘synthetic’ view, which focus on the coalescence of multiple variables. The dual concept of sleep organization evolved stepwise in parallel to the rapid growth of neurophysiological knowledge and techniques in the first half of the 20th century, culminating in the discovery of REM sleep.