Theories about sleep and EEG-sleep stages

Recent publications on the formation of sleep theories and theories on the importance of individual sleep stages are reviewed. It seems evident that sleep is regulated by two processes as documented in the theories of Borbely. The light-darkness-triggered 24 1/2-25h rhythm pro cess C induced by the suprachiasmatic nuclei, and the homoeostatic process 5, induced by the work/wake time and the kind of activity done during wakin g. in addition, there is the 90-min REM/NREM cycle during day and night, ac cording to Hobson caused by changing firing rates in the peri-brachial pons . While processes S and Care plausible for the theory of restorative function s of sleep, for the REM/NREM cycle no satisfactory theory is available. The restorative functions of sleep include muscle resting, decreased thalamic activity as well as periodic process C-dependent and process S-dependent pr oduction of hormones and proteins. The 90-min cycle of REM/NREM sleep is most likely necessary for memory cons olidation, error correction and nerve fibre sprouting. However, there are n o detailed theories available as to when and where exactly and through whic h mechanisms during sleep this would happen. Many publications as reviewed by Sejnowski and Destexhe describe the neurophysiological and biochemical m echanisms during SWS and REM; however, these findings do not result in a ho listic theory about the importance or the mechanisms of REM/NREM cycle inte grated into the 2-process model of Borbely.