This chapter begins with an overview of the neural systems involved in vigilance state regulation. The first successfully recorded electrical activity of the human brain highlighted that the profile of electroencephalograms is changed across the vigilance states. Wakefulness is characterized by a cortical EEG profile of low-voltage, fast/high-frequency field potentials of the alpha, beta, high beta, and gamma spectral range. Non-rapid eye movement (NREM) sleep may be classified into three stages according to recent criteria of the American Academy of Sleep Medicine (AASM). The field of sleep medicine has seen a recent investigational surge, due to the development of novel experimental techniques, as well as an increased public awareness of sleep disorders and their implications. All human life, and indeed the life of most animals, is shaped by periods of wakefulness and sleep, and thus knowledge of the underlying mechanisms is of great biological, social, and medical significance.