Why do we sleep? Although we spend about one-third of our lives asleep, the roles of sleep remain largely unclear. Studies using animal models have shown that sleep contributes to homeostasis and plasticity, which are critical for adapting to changing environments. In humans, advances in neuroimaging techniques have enabled researchers to measure brain activity during sleep with increasingly high spatiotemporal resolution. The brain undergoes dynamic changes that contribute to distinct aspects of learning during both non-rapid eye movement (NREM) sleep and rapid eye movement (REM) sleep. When sleeping in a new and unfamiliar environment, asymmetric sleep patterns emerge. In this chapter, we first review basic characteristics of sleep, measured by polysomnography including EEG, which will be followed by recent findings on how sleep contributes to memory and learning, and their link to metabolite waste clearance, using EEG concurrently with various neuroimaging methods.