When speculating about the function of dreaming, some clarifications about the level of explanation are necessary. By definition (e.g., Windt 2010), dreaming is a phenomenon occurring during sleep. In an account of biological realism (Revonsuo 2006), the function of dreaming cannot be discussed independently from the neurophysiology of sleep. Even if the phenomenology of dreaming serves a function that can be conceptually (and maybe evolutionarily) differentiated from the original function realized by its physiological correlates, this function is not independent from the neurophysiology of sleep and its specific functions: if the neurophysiological functions change their mechanisms, this would also affect the phenomenological aspects of dreaming—philosophically speaking, phenomenal properties of dreaming supervene on neurophysiological properties of sleep. However, neither can the function of dreaming be equated with the function of sleep, since there are functions of sleep for which it is rather unlikely that any phenomenological aspects play a role, e.g., myelin sheath proliferation (Bellesi et al. 2013); synaptic downscaling (Tononi & Cirelli 2006); metabolite clearance (Xie et al. 2013); or general metabolic (Morselli et al. 2012) and immunological functions (Besedovsky et al. 2012). There are also functions of sleep that might be described conceptually without referring to phenomenal aspects, but in fact happen to be biologically associated with dream mentation, e.g., physiological microprocesses underlying memory consolidation (see below). And in these cases, one can differentiate dream phenomenology and sleep physiology on a conceptual, but not biological level—unless one adopts a radically dualistic approach, that is. Hence, speaking of the function of dreaming —in contrast to the function of sleep more generally—always implies both phenomenological and physiological aspects.
When considering the neurophysiology of dreaming, coarse sleep stages as defined by classical polysomnography have been the prime targets of investigation. Among these, REM sleep harbors the most prototypical dreams, with a story-like dream narrative including interactive visuomotor hallucinations and often intense emotions. In addition, REM sleep dreams can be most elegantly related to their neurophysiological correlates (Hobson & Pace-Schott 2002). Nevertheless, dream-like mentation can be found in in all sleep stages (Nielsen 2000), and hence also the neurophysiology of other sleep stages has to be taken into account when investigating the function of dreaming. In conclusion, when speculating about the function of dreaming, all those REM and NREM sleep functions have to be considered that can reasonably be expected to be associated with phenomenal aspects. In the following, I will highlight four clusters of such sleep functions.