“Sleep remained one of the last great biological mysteries.” 

Sleep initially appears to be an unnecessary biological phenomenon because when you are sleep you cannot complete key tasks that produce evolutionary advantages (e.g., gather food, find a mate and reproduce, and protect offspring). The fact that all animals sleep and that sleep takes 25 to 30 years from our lives illustrates that it is critical to humanity’s evolution. Sleep remained shrouded in mystery, however, until the last two decades with the advent of MRI scanning. For this reason, Walker refers to sleep as “one of the last great biological mysteries.” 

“Everyone generates a circadian rhythm (circa, meaning ‘around,’ and dian, derivative of diam, meaning ‘day’). Indeed, every living creature on the planet with a life span of more than several days generates this natural cycle. The internal twenty-four-hour clock within your brain communicates its daily circadian rhythm signal to every other region of your brain and every organ in your body.” 

This passage provides the definition of the circadian rhythm, which controls the sleep-wake cycle. As the formulation indicates, the circadian rhythm rises and falls every 24 hours. While all individuals generate the circadian rhythm, the timing varies. This explains why some people are night owls and others morning larks. 

“When it comes to information processing, think of the wake state principally as reception (experiencing and constantly learning the world around you), NREM sleep as reflection (storing and strengthening those raw ingredients of new facts and skills), and REM sleep as integration (interconnecting these raw ingredients with each other, with all past experiences, and, in doing so, building an ever more accurate model of how the world works, including innovative insights and problem-solving abilities.” 

This passage provides the definition of the wake and NREM and REM sleep states. The reception of information and experiences from the outside world characterizes the waking state. The deep NREM slow-wave sleep transfers these recent experiences from a short-term storage site to a long-term storage site.