How is the brain activated in sleep

The reflex doctrine could not, however, help the pioneer sleep and dream scientists very much, because no link could be established between the activity of circuits of neurons (neuronal circuits) and the EEG. It had long been assumed that the EEG was the register of voltage changes in the brain (i.e. cerebral action potentials), although this could not explain the patterns of the EEG seen in sleep (e.g. spindles and slow waves), unless neuronal activity was continuous, i.e. spontaneous, as well as reflexive. Consequently, work at the cellular and EEG levels proceeded along entirely separate but parallel tracks, similar to those that Descartes thought God had used to set mind and body in perfect but independent motion. Cartesian duality dies slowly it is still alive and well in most of us because we cannot yet see how a physical object, the brain, can have subjective experience. This is the so-called hard problem of philosophy. 

How and why is the limbic forebrain selectively activated in REM so as to produce this kind of affective experience The how answer is that in addition to its major cholinergic innervation from the basal forebrain the limbic area receives a direct projection from the dorsolateral pontine tegmentum, where the cholinergic neurons that become selectively active in REM sleep are found in animals. PET studies can t yet tell us if this particular brain stem region is selectively activated in human REM sleep, but we hypothesize that such a homology is likely. 

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