Why do nearly all animals sleep? A new Perspective in Brain Medicine gathers evidence from neuroimaging, electrophysiology, and machine learning to propose an answer that sits above the familiar ones. The authors synthesize existing research to argue that sleep operates as a system-level resilience mechanism, stabilizing large-scale brain dynamics, curbing maladaptive over-specialization, and preserving efficient information flow. NREM and REM stages emerge as coordinated phases of repair and reconfiguration. The same logic, the authors suggest, may extend to artificial neural networks, where structured offline phases could ease catastrophic forgetting. Sleep, in this reading, is active maintenance rather than passive rest.