Emil Kraepelin's concepts of the phenomenology and physiology of sleep: The first systematic description of chronotypes

Emil Kraepelin is considered one of the most influential psychiatrists ever. His research on sleep, however, has received little attention to date. Therefore, Kraepelin's published work was reviewed, statements on the topic “sleep” identified, historically contextualized and compared with current knowledge. His assumptions on the “physiology of sleep” are rather speculative and not substantiated by own research. The opposite is true for his findings on the “phenomenology of sleep”. For example, his results on sleep depth and sleep stages are not only in overall good agreement with current concepts, but also based on sleep studies. Of special relevance are his findings on chronotypes: Based on empirical clinical studies, neuropsychological experiments and sleep studies, Kraepelin systematically described a morning and evening disposition and intermediate types on the basis of the maximum physical capacity, cognitive ability and alertness/sleepiness during the day as well as sleeping habits. In fact, this concept is basically still valid and these aspects are being captured in morningness-eveningness scales. Our findings challenge the common notion that Nathaniel Kleitman was the first to systematically describe the morningness-eveningness dimension. Overall, we deem Kraepelin's contributions to sleep research, compiled here for the first time, well worth being acknowledged by modern sleep research.