you may also check out the quite logical and convincing "Visceral Theory of Sleep":
> The visceral hypothesis suggests that during sleep, the central nervous system, particularly the cerebral cortex, switches from analyzing exteroceptive information to analyzing signals arriving from interoceptors distributed throughout all the systems of the body
> The visceral hypothesis suggests that during sleep, the central nervous system, particularly the cerebral cortex, switches from analyzing exteroceptive information to analyzing signals arriving from interoceptors distributed throughout all the systems of the body