This chapter focuses on the neurotransmitter and neuromodulator systems involved in the regulation of wakefulness and sleep as well as the neurochemical responses to sleep loss. Wakefulness, rapid eye movement (REM), and non-REM (NREM) states were originally defined in mammals using measures of skull surface electrical brain activity, skeletal muscle activity, and eye movements. The two primary factors that determine the degree of human vigilance and sleepiness are the duration of prior wakefulness and circadian influences. Increases in homeostatic sleep need are associated with subjective sleepiness, objective sleepiness, diminished neurocognitive function, as well as neurochemical and neurophysiological changes. The ascending reticular activating system (ARAS) is comprised of the brainstem reticular formation and its ascending projections responsible for cortical activation and wakefulness. Electrophysiological and neurochemical data indicate that highest levels of orexinergic activity occur during active wakefulness, and greatly reduced activity is seen during NREM and REM sleep.