This book investigates the fundamental biological purpose of sleep in humans and other mammals, questioning why such a vulnerable state evolved. Author James Horne, a prominent sleep researcher, utilizes comparative biological data and evolutionary theory to challenge common assumptions about sleep's restorative functions. He argues that sleep is primarily a mechanism for brain recovery rather than a simple period of physical rest, providing a framework that distinguishes between core sleep and optional sleep.

Experts recognize this work as a significant contribution to the biological understanding of sleep, particularly for its focus on evolutionary function over physiological mechanics. Readers frequently note that the prose remains accessible to those with a basic background in biology while maintaining a rigorous scientific focus.