Historically, researchers studying the field of sleep have been divided into two main groups. The first group of sleep studies was based on behaviors and behavioral sleep. The second group of researchers based their studies on electroencephalogram EEG patterns and changes. With technologic advancements, a third group of researchers is studying sleep using active brain-imaging techniques. Most events that are seen on brain imaging correlate with the EEG pattern, so that they relate to each other in time and state. Unfortunately, behavioral states do not correlate necessarily with the EEG changes and vary greatly between animal species. This article focuses on sleep stages and behaviors that are associated with characteristic EEG patterns.