Sleep is usually considered a resting state .In reality, this conception does not necessarily agree with some of the physiological processes during sleep. Neurons in most parts of the brain remainactive during sleep and the brain expends much energy with this neural activity during sleep. It is easy to conceive that neurons responsible for autonomic functions such as respiration remain active in both sleep and waking, but neurons in other parts of the brain also remain active, often in a highly synchronous and rhythmic manner.
Studies in humans half a century ago first demonstrated that sleep occurs in two distinct states: rapid eye movement (REM) sleep and non-REM (NREM) sleep. These sleep states exist in human fetuses and newborns, although their earliest age of appearance is controversial. Studies using chronically catheterized fetal animals and imaging of the human fetus have emphasized the similarities between fetal and postnatal sleep states. The current concept regarding REM sleep is that there is a controlling network composed of several areas of the forebrain and that brainstem structures may be responsible for its final expression. With respect to the development of NREM sleep, it also requires the establishment of a specific network of excitatory and inhibitory neuralcomponents, that includes the formation of thalamocortical and intracortical patterns ofinnervation.