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 or 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'. The Soviets managed to recruit several people after World War I who reached high positions in Britain and in the U.S.
Those sleeper agents who have been discovered have often been natives of the target country who moved elsewhere in early life and been 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.
