sleeper

A person who is asleep or who sleeps in a specified way.

"He's a very light sleeper."

A secret agent or terrorist who remains inactive until activated for a mission.

"The intelligence agency uncovered a sleeper cell."

A heavy wooden or concrete beam supporting railway tracks; a railway tie.

"The old railway sleepers needed replacing."

An item, film, or product that achieves unexpected success after a slow start.

"The film was a real sleeper hit."