This chapter examines the multifaceted nature of sleep and its disorders, focusing on the macro- and microstructure of sleep, the global and local aspects of sleep electroencephalogram (EEG), and the evolution of sleep EEG throughout the lifespan. We explore the organization of both non-rapid eye movement (NREM) and rapid eye movement (REM) sleep stages and how dynamic processes such as arousal fluctuations underlie sleep stability and regional brain activity. The chapter also explores the features of polysomnographic recordings and their relevance to diagnosing various sleep disorders, including narcolepsy, insomnia, REM sleep behavior disorder, sleep-related hypermotor epilepsy, and NREM parasomnias. Through this investigation, this chapter offers a general and succinct overview of the key aspects of sleep and its disorders, focusing on what EEG reveals about sleep regulation and sleep disorders.