But though doctors have been putting people under for more than 150 years, what happens in the brain during general anesthesia is a mystery. Scientists don’t know much about the extent to which these drugs tap into the same brain circuitry we use when we sleep, or how being anesthetized differs from other ways of losing consciousness, such as slipping into a coma following an injury. Are parts of the brain truly shutting off, or do they simply stop communicating with each other? How is being anesthetized different from a state of hypnosis or deep meditation? And what happens in the brain in the transition between consciousness and unconsciousness? “We know we can get you in and out of this safely,” Brown says, “but we still can’t quite tell you how it works.”