In Chapter 1, we address the sleep/wake cycle and how this circadian behavior is affected by a myriad of neurotransmitter systems. We also discuss the molecular clock, a series of interacting transcription factors that influence sleep and many other physiological processes, and how genetic variation in molecular clock components may influence the sleep/wake cycle as well as cardiometabolic health, mental illness propensity, and risk of cancer. We hope that this chapter will convey the importance of sleep to whole body wellness so that readers will more enthusiastically appreciate the need for accurate diagnosis (Chapter 2) and appropriate treatment (Chapter 3) when the sleep/wake cycle is disrupted.