In 1888, Eduard Robert Michelson (1861-1944), a student of the German psychiatrist Emil Kraepelin at the university clinic of Dorpat (Tartu, Estonia), established a sleep laboratory in which he conducted a fundamental and innovative study about the physiology of sleep regulation. Based on the then current theoretical concepts and methodological techniques of Wundtian experimental psychology, and Kraepelin's research strategy, Michelson, for the first time, was able to describe a "very strange phenomenon" of human sleep - a "remarkable periodicity" of the "sleep depth curve." Furthermore, Michelson postulated that this within sleep periodicity should not be explained as an effect of external stimuli but rather of "antagonistic" physiological processes. Unfortunately, Michelson's publication of 1891 fell almost into oblivion as contemporary theories of sleep could not offer an explanation for his findings. Nevertheless, Michelson's "Untersuchungen über die Tiefe des Schlafes" should be considered as one of the key studies in the development of sleep research in the 19th century and a pioneer description of within sleep periodicity.