In the process of evolution, owing to progressing the development of the vertebrate brain, the functional activity is changed both in the structures generating and regulating sleep and in the sleeping structures, i.e., on one hand, there are developed the structures that need sleep and are based on telencephalon, but, on the other hand, there are developed structures of diencephalon, mesencephalon, and rhombencephalon that provide regulation of SWS and FWS. (598)
This piecewise evolution of the mammalian brain resulted in two main waking types; diurnal and nocturnal. During the diurnal activity, the sensory-motor cortex is constantly under stimulation by the midbrain. During the nocturnal activity, this neural stimulation is inhibited by stimulation from the diencephalon. In essence, sleep can be thought of as the process whereby the diencephalon inhibits the activity of the mesencephalon. It is the balance of activity between these two components that produces sleep and regulates REM and NREM patterns. This neurological structure plays an essential part in the evolutionary development of mammalian sleep patterns.