Sleeping gas is an oneirogenic general anaesthetic that is used to put subjects into a state in which they are not conscious of what is happening around them. Incapacitating agent is a related general term for "knockout gases" or "KO gas" that ideally render a person unable to harm themselves or others, regardless of consciousness. Most sleeping gases have undesirable side effects, or are effective at doses that approach toxicity.
Examples of modern volatile anaesthetics that may be considered sleeping gases are halothane vapour (Fluothane), methyl propyl ether (Neothyl), methoxyflurane (Penthrane), and the undisclosed fentanyl derivative delivery system used by the FSB in the Moscow theater hostage crisis.
Sleep gas is extremely rare, only the wealthiest Baron's possess any.