A sleeper agent is a spy who is placed in a target country or organization, not to undertake an immediate mission, but rather to act as a potential asset if activated. Sleeper agents are popular plot devices in fiction, in particular espionage fiction and science fiction.
Sleeper agents in espionage 
In espionage, a sleeper agent is one who has infiltrated into the target country and 'gone to sleep', sometimes for many years. That is, he or she does nothing to communicate with his or her sponsor nor any existing agents, nor to obtain information beyond that in public sources. They can also be referred to as 'deep cover' agents. They acquire jobs and identities—ideally ones which will prove useful in the future—and attempt to blend into everyday life as normal citizens. Counter-espionage agencies in the target country cannot, in practice, closely watch all of those who might possibly have been recruited some time before.
In a sense, the best sleeper agents are those who do not need to be paid by the sponsor as they are able to earn enough money to finance themselves. This avoids any possibly traceable payments from abroad. In such cases, it is possible that the sleeper agent might be successful enough to become what is sometimes termed an 'agent of influence'.
Sleeper agents who have been discovered have often been natives of the target country who moved elsewhere in early life and were co-opted (perhaps for ideological or ethnic reasons) before returning to the target country. This is valuable to the sponsor as the sleeper's language and other skills can be those of a 'native' and thus less likely to trigger suspicion.
Choosing and inserting sleeper agents has often posed difficulties as it is uncertain which target will be appropriate some years in the future. If the sponsor government (or its policies) change after the sleeper has been inserted, the sleeper might be found to have been planted in the wrong target.
- Otto Kuehn and family were installed in Hawaii by the German Abwehr, before World War II, to work for Japanese intelligence. Kuehn, and his family, aided the Japanese in the period before the Attack on Pearl Harbor. They seem to have been less than effective.
- Kim Philby was recruited by the Soviets while at university and may have been a sleeper agent for some years until going to work for the British government. By the end of WWII, he was operating as the liaison between the British Secret Intelligence Service and several U.S. intelligence operations. He was an agent of influence by then, but had not been a sleeper agent for several years.
- The so-called Illegals Program is an alleged network of sleeper spies planted in the U.S. by the Russian Foreign Intelligence Service. The ongoing, multi-year investigation culminated in June 2010 with the filing of charges and the arrest of 10 suspects in the U.S. and another in Cyprus.
Fictional sleeper agents 
In fiction, particularly science fiction, sleeper agents fall into two categories. The first is an extension of the real world sleeper agent where an enemy agent is substituted for a person already in a trusted position. The second and more common category involve people who have been subjected to mind control techniques, such as drugs, torture, psychological conditioning, implanted devices, and even telepathic manipulation who then are either released, or allowed to escape back to friendly territory. These sleeper agents are then used by enemy forces to spy, to conduct sabotage, to assassinate certain targets, or for other operations the enemy has in mind for them. During these outbreaks, the sleeper agent doesn't normally know what he/she is doing.
The substitution sleeper agent was often surgically altered to appear as someone else but more recent versions tend toward androids or clones. Or the agent may have been an infiltrator from the start but brainwashed to believe they are the real thing until activation.
Activation of the second kind of sleeper is, at least in novels and stories, done by approaching the agent and uttering a long ago memorized password or pass phrase, or by mailing a postcard with a significant picture to the sleeper. Once a sleeper becomes active, counter intelligence agencies can, at least in principle, become aware of the sleeper as intelligence is collected and transmitted, as instructions are passed, and so on.
In popular culture 
There are a number of examples of sleeper agents found in science fiction and other forms of entertainment. Many times the unveiling of a sleeper agent is an important part of the plot and acts as surprise element and a plot twist. Some examples of sleeper agents include:
- One of the earlier uses of the second type of sleeper agents in fiction is in Richard Condon's 1959 novel, The Manchurian Candidate, which has twice been adapted to film. Both the original and the remake is about a group of people 'programmed' to be sleeper agents. One of the sleeper agents is part of a Presidential election campaign, which if won will produce a Vice President controlled by sinister forces. One of his fellows would then be ordered to assassinate the President, allowing these forces to control the Executive Branch of Government.
- Another early use of sleeper agents is in the 1977 Charles Bronson film "Telefon (film)." Bronson plays an unwitting KGB agent whose trigger phrase is borrowed from Robert Frost's poem, "Stopping by Woods on a Snowy Evening."
- In the show 24, the villain Habib Marwan commands a network of sleeper agents. His hierarchy is complex and secretive, with families only being able to identify the rest of the family (as well as Marwan) as the only members of their "cell".
- Both the book and film Eye of the Needle demonstrate how a sleeper agent has to operate within a host country - in this case, the German agent Henry Faber in World War II Britain. This story portrays points of view of both the agent and the British intelligence services as the hunt for the sleeper spy continues across Great Britain. Faber discovers the true nature of Operation Fortitude and tries to get the information to his home country.
- The BBC mini series Sleepers centered around two Russian sleeper agents who had so fully integrated in British society that they were keen to avoid being brought back to the Soviet Union.
- The seventh series of BBC spy drama Spooks involved an MI6 operation to put a network of sleeper agents in post-Soviet Russia as the Berlin Wall fell, and a Russian counter-operation to infiltrate Britain. The finale involved a Russian sleeper detonating a suitcase nuke in the heart of London.
- In the Battlestar Galactica TV miniseries (2004), Raptor pilot Sharon Valerii is a sleeper Cylon. She is unaware of her true identity until activated.
- The 1987 movie No Way Out is a cold war themed fiction of US government frantically searching for an alleged mole whose is elusiveness is revealed in his being a successful sleeper agent.
- The 1988 movie Little Nikita (starring Sidney Poitier and River Phoenix) revolves around a teenager in America finding out that his parents are actually Soviet sleeper agents.
- Martin Scorsese's 2010 movie Shutter Island involved an imaginary plot to create sleeper agents at a Mental Institution.
- In the 2010 film Salt, Evelyn Salt was exposed as a sleeper agent but after finding out her husband was killed by the hands of her own people, she decides to annihilate the missions she was supposed to execute.
- In the 2010 game Call of Duty Black Ops, the main character Alex Mason is turned into a Russian sleeper agent while being captured as a P.O.W. at the real-life gulag Vorkuta in the northern Soviet Union so that the Soviet Union can attack the United States with other sleeper agents, including Mason with the fictional Nova 6 nerve agent.
- Garage-rock band Sleeper Agent derives its name from Cylon sleeper agents in the show Battlestar Galactica.
- In the comic series Assassin's Creed: The Fall, the main character Daniel Cross was a sleeper agent of the Templar Order (Abstergo Industries) assigned to kill the Mentor of the Assassin Order, succeeding in the year 2000 in Dubai.
- In Assassin's Creed Revelations DLC Lost Archive Lucy Stillman was reveled as a sleeper agent, or a triple agent, during a conversation that Clay Kaczmarek (Subject 16) over heard between Lucy and Warren Vidic
- 2012 Tamil movie Thuppakki's plot revolves around the hero Jagdish (Vijay), a captain in the Indian Army fighting a set of sleeper agents terrorizing Mumbai.
- TV series The Americans is a Cold War drama. The premise is that a group of Soviet KGB officers have been trained to impersonate American citizens, so that each one can become a sleeper agent, with a cover which may even include an unwitting spouse and family.
- In the 2007 game Bioshock, the main protagonist, Jack, is revealed to be a sleeper agent. When a command is given with the phrase "Would you kindly?" included in it, Jack immediately does what is said with no questions asked. The first instance of it is at the beginning of the game, forcing him to take a plane down to begin the events of the game.
See also