Abstract
This paper presents a brief historical review of the beginning of scientific studies on sleep research and evaluation of the electrophysiological basics. Subsequently, hypotheses are presented and discussed on phylogenesis and function of NONREM- and REM-sleep as well as aspects of the regulation of energy conservation and temperature. Sleep may have first evolved out of rest/activity cycles. Later on, with the evolution of complex brains, sleep may also assume importance in development and maintenance of these structures. Hence, special emphasis is placed on the influence of NONREM and REM sleep on the reorganisation of neuronal networks.