2 Overview of Normal Sleep

The history of sleep medicine and sleep research is a history of remarkable progress and remarkable ignorance. In the 1940s and 1950s, sleep had been in the forefront of neuroscience, and then again in the late 1990s, there has been a resurgence of our understanding of the neurobiology of sleep. Sleeping and waking brain cycles can now be studied by sophisticated neuroimaging techniques which have shown remarkable progress by mapping different areas of the brain during sleep states and stages. Electrophysiological research has shown that even a single neuron sleeps as evidenced by the electrophysiological correlates of sleep– waking at the cellular (single cell) level. Despite recent progress, we are still groping for answers to two fundamental questions: What is sleep? and why do we sleep? Sleep is not simply an absence of wakefulness and perception nor is it just a suspension of sensorial processes but is a result of a combination of a passive withdrawal of afferent stimuli to the brain and functional activation of certain neurons in selective brain areas. The more important question, however, is “How do we stay awake? ” In the mother’s womb, we were all asleep, and we wake up the moment we are born. But even in the newborn, sleep occupies 16 out of 24 h. Therefore, the fundamental inquiry should be directed at the mechanism of wakefulness. Since the dawn of civilization, the mysteries of sleep have intrigued poets, artists, philosophers, and mythologists. The fascination with sleep is reflected in the literature, folklore, religion, andmedicine.Upanishad [1, 2] (circa 1000B.C.), the ancient Indian textbook of philosophy, sought to divide human existence into four states: the waking, the dreaming, the deep dreamless sleep, and the superconscious (“the very self”). This is reminiscent of modern classification of three states of existence (see later). One finds the description of pathologic sleepiness (possibly a case of Kleine–Levin syndrome) in the mythologic character Kumbhakarna in the great Indian epic Ramayana [3, 4] (circa 1000 B.C.). Kumbhakarnawould sleep for months at a time, then get up to eat and drink voraciously before falling asleep again (Fig. 2.1). The ancient Chinese believed in two basic principles of life: yang, the active, light, and positive; and yin, the passive, dark, and negative. The yin– yang concept, originated with Fu Hsi (circa 2900 B.C.), has since become a symbol for sleep and wakefulness [5]. Throughout the literature, a close relationship between sleep and death has been perceived, but the rapid reversibility of sleep episodes differentiates sleep from coma and death. There are myriad references to sleep, death, and dream in poetic and religiouswritings, including the following quotations: “The deepest sleep resembles death” (The Bible, I Samuel 26:12); “sleep and death are similar . sleep is one-sixtieth [i.e., one piece] of death” (The Talmud, Berachoth 576); “There she [Aphrodite] met sleep, the brother of death” (Homer’s Iliad, circa 700 B.C.); “To sleep perchance to dream. For in that sleep of death what dreams may come? ” (Shakespeare’sHamlet); “How wonderful is death; Death and his brother sleep” (Shelly’s “Queen Mab”). The 3 major behavioral states in human—wakefulness, non-rapid eye movement (NREM), and rapid eye movement (REM) sleep—are three basic biological processes that have independent functions and controls. The reader should consult Borbely’s monograph Secrets of Sleep [2] for an interesting historical introduction to sleep. S. Chokroverty (&) School of Graduate Medical Education, Seton Hall University, South Orange, NJ, USA e-mail: schok@att.net