Based on data from animals with prenatal brain development, there is evidence for prenatal sleep states development. Of note, regional blood flow within the brain increases during REM sleep, especially in areas thought to be involved with the generation of this sleep state, cerebral leucine metabolism correlates to sleep states, suggesting that protein synthesis and degradation processes within the brain are modified by them, and cerebral blood flow and oxygen delivery are higher during spontaneous and carbachol-induced REM sleep relative to spontaneous and scopolamine-induced NREM sleep, indicating that pharmacologic manipulation induced fetal sleep states behaviorally, electrophysiologically and metabolically. In addition, in newborn lambs, the cerebral metabolic rate of oxygen consumption is as high during REM sleep as during wakefulness. As awhole, these studies emphasize that comparisons of fetal and adult sleep states in developmental profile and pharmacological responses may be species specific.