Dear Colleagues,

Understanding consciousness and its evolution in higher-intelligence beings is among the most sought-after subjects desired and explored by intellectuals from every field of studies, including philosophers, physicists, chemists, biologists, neuroscientists, and so on. Since the evolution of modern neurobiological science, consciousness has been classified primarily into two states: wakefulness and sleep. Subsequently, with the progress in knowledge during mid-twentieth century, sleep has been divided into rapid eye movement (REM) sleep and non-REM sleep. The REM sleep is unique in the sense that, although one remains behaviourally asleep, the brain expresses electrophysiological signals comparable to waking. This unique state of REM sleep does not have a voluntary control; most of the dreams appear involuntarily and intermittently during this state. Since the discovery of REM sleep in 1953, in the following seven decades, consistent and significant efforts have been made to understand the biology, neural regulation, and functions of REM sleep. It is known that, at least in the higher vertebrates, REM sleep is an autonomically regulated, instinct behaviour; it is affected in almost all diseases, its loss affects almost all physiological processes, and its sustained loss often become fatal. Notwithstanding, our understanding about REM sleep is far from satisfactory, e.g., we do not know any marker to identify its loss, we cannot control (increase or decrease) it exclusively in a predictive manner; additionally, it is yet unclear if it is a state of consciousness, as well as why and how it modulates consciousness and cognitive ability. Furthermore, during REM sleep, as the brain appears to remain awake during behavioural deep sleep, a detailed understanding of neural and other regulation of REM sleep might facilitate our understanding of consciousness. From time to time, there have been isolated reviews and monographs focusing on various aspects of REM sleep; the most comprehensive one was more than a decade ago. Hence, it is high time that, in response to the significant interest, we produce a dedicated volume dealing with review of advances in the knowledge of REM sleep in the past seven decades, present knowledge as well as challenges and way forward. 

Prof. Dr. Birendra Nath Mallick
Prof. Dr. Noor Alam
Guest Editors

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