Functional Aspects of the Sleep EEG

Only the recording of brain electrical activity (electroencephalogram, EEG) during sleep revealed that sleep is an active process that is tightly regulated. Thus, every night we cycle through a seemingly predefined series of discrete states (NREM and REM sleep) each with its characteristic EEG activity pattern. Sleep need is also regulated and depends to a large extent on how long we stay awake. Moreover, the longer we stay awake the more intense sleep becomes. The quantitative analysis of the sleep EEG led to the discovery of slow-wave activity (EEG power below 4.5 Hz) as a marker of sleep intensity, which closely reflects the homeostatic regulation of sleep. A recent hypothesis about the function of sleep attributes a central role in the maintenance of cortical connections, i.e. synapses, to this major EEG characteristic of NREM sleep. According to the synaptic homeostasis hypothesis, NREM sleep rich of slow waves favours the cleaning or recalibration of cortical connections, making them more efficient for signal transduction. Such increased efficiency of synapses might be responsible for the widely observed sleep dependent performance improvements. The clinical impact of the synaptic homeostasis hypothesis is exemplified by a potential relationship between the increased seizure risk during deep NREM sleep and changes in cortical signal transduction.