The PDF "Why We Sleep" by Matthew Walker explains that sleep serves multiple vital functions for both the brain and the body. Sleep enhances learning, memory, decision-making, emotional regulation, and creativity in the brain. It also supports physical health by strengthening the immune system, regulating metabolism, controlling appetite, maintaining gut health, and promoting cardiovascular fitness. Sleep helps to restore and optimize nearly every major system of the body and brain, making it essential for overall health and longevity. The book also highlights that sleep is controlled by the balance between the circadian rhythm (our internal 24-hour clock) and the chemical sleep pressure that builds the longer we stay awake. Insufficient sleep impairs nearly all of these processes and dramatically raises the risk of serious health problems including cancer and heart disease.