Many neuroscientists and philosophers today think of dreamless sleep (see glossary) as a blackout state in which consciousness is entirely absent. Indeed, they often appeal to this apparent fact in order to define consciousness:
Everybody knows what consciousness is: it is what vanishes every night when we fall into a dreamless sleep and reappears when we wake up or when we dream. (Tononi 2008, p. 216)
Consciousness consists of inner, qualitative, subjective states and processes of sentience and awareness. Consciousness, so defined, begins when we wake in the morning from a dreamless sleep and continues until we fall asleep again, die, go into a coma, or otherwise become “unconscious”. (Searle 2000, p. 559)
I will call the view that consciousness vanishes or ceases in dreamless sleep the default view of the relationship between consciousness and dreamless sleep. One aim of this paper is to argue that the default view is not as obvious or strong as it is often thought to be. Another aim is to propose that we need a finer taxonomy of sleep states than that which sleep science currently employs, in order to allow for the possibility of states or phases of dreamless sleep in which consciousness is present. There are forceful reasons, if not decisive ones, for describing certain kinds of dreamless sleep as modes of consciousness rather than as the absence of consciousness. These reasons derive from the debate about dreamless sleep between the Advaita Vedānta and Nyāya schools of Indian philosophy (see glossary). Examining this debate in the light of cognitive science raises important conceptual and methodological issues for the cognitive neuroscience of consciousness. Furthermore, considerations about sleep drawn from Indian philosophy suggest new experimental questions and protocols for the cognitive neuroscience of sleep and consciousness. By weaving together these different traditions—Western cognitive science and Indian philosophy—I hope to show the value of cross-cultural philosophy of mind for cognitive science.