Sleep onset process as an altered state of consciousness

Falling asleep is a link between two general states of consciousness, wakefulness and sleep. During the complex process of the wake to sleep transition, various electrophysiological, cognitive and behavioural alterations take place, all linked to states of consciousness. Together, these states have been interpreted as the hypnagogic state. It is no longer believed that sleep onset just acts as a buffer between the awake and sleep processes characterised by the gain and loss of sensory functioning, respectively. On the contrary, the sleep onset phenomenon addresses many issues surrounding some of the most common sleep and mental disorders. What is the moment of sleep onset? How can sleep onset be characterised from the brain, heart and respiration activity during sleep in humans? These are some of the questions that have no clear and de?nite answers. Neural synchrony and its interaction and coupling with cardio-respiratory synchrony is very much related to its role as a mechanism for integrating brain, heart and respiration, which are all responsible for consciousness, in both humans and animals. The phase synchrony between neural, cardio and respiratory activities and environment provides a signature of subjective experiences, linked to various states of consciousness. Technological advances have led to a gradual shift in the approach adopted by the scienti?c community towards unravelling and understanding this sleep onset phenomenon as an altered state of consciousness. This chapter presents research from the scienti?c community, explores some new objective indices that can be adopted to measure sleep onset, and demonstrates how sleep onset can be voluntarily altered and induced using biofeedback technology.