Sleep is an almost ubiquitous phenomenon within the animal kingdom, existing in all higher and many lower species. The specific function of sleep, however, is still an enigma: sleep helps an organism to save energy through extended periods of inactivity, yet at the same time leaves it in a potentially dangerous state of non-responsiveness. While several possible functions of sleep have been discussed in recent years (Frank 2006; Vassalli & Dijk 2009), the function of dreaming might be seen as an even bigger mystery: the hyper-realistic imagery experienced during dreaming does not inform the organism about its current environment, and the virtual motor activity processed in interaction with these hallucinations is not executed to affect the external world—or even worse, in pathological conditions like REM sleep behavior disorder it is, thereby threatening the health of the dreamer and her bed partner. After awakening from a dream, the often emotionally-toned preoccupation with the dream narrative can confuse the dreamer and distract her from potentially dangerous conditions in the real world.
An increasingly widespread idea is that the function of dreaming consists in the simulation of waking life. In a variation of their threat simulation theory (TST; Revonsuo 1995, 2000), Revonsuo et al. (this collection) now propose a social simulation theory of dreaming (SST), according to which dream function could best be characterized as simulating social reality. Considering the social nature of most of our dreams, SST is an intuitively plausible approach, and Revonsuo et al. review a number of studies that provide support for SST. Nevertheless, several questions remain to be clarified: is the prime function of dreaming threat simulation or social simulation—or something completely different? What is the relationship between the various proposed functions of sleep and dreaming, including TST and SST? If the TST and SST turn out not to be the sole or even prime functions of dreaming, do they nevertheless provide unique insights into the function of dreaming?
In this commentary, I shall review several widely propagated functions of sleep and dreaming. I shall then compare these functions with the social and threat simulation functions of dreaming, and finally discuss why and in which regard these two functions might be special. I shall argue that the merit of TST and SST is not the conclusive explanation of the function of dreaming—which I consider a multifunctional state—, but that they are the only candidates among the variety of dream functions that are capable of explaining a striking feature of most dreams: obliviousness towards the current state of mind.