 Different theories on the functions of dreaming have been proposed and the debate is ongoing. An important distinction is between the functions of sleep stages and the functions of dreaming. Well-documented functions of REM sleep include thermoregulation and the development of cortical structures in birds and mammals, as well as neurotransmitter repletion, the reconstruction and maintenance of little-used brain circuits, the structural development of the brain in early developmental phases, as well as the preparation of a repertoire of reflexive or instinctive behaviors (Hobson 2009). Yet none of these functions are obviously linked to dreaming. An exception is protoconsciousness theory, in which REM sleep plays an important role in foetal development by providing a virtual world model  even before the emergence of full-blown consciousness (Hobson 2009: 808).