Definitions of sleeper:

• noun:   an unexpected hit

  Example: "That movie was the sleeper of the summer"

• noun:   tropical fish that resembles a goby and rests quietly on the bottom in shallow water
• noun:   a piece of furniture that can be opened up into a bed
• noun:   a rester who is sleeping
• noun:   an unexpected achiever of success

  Example: "The winner was a true sleeper--no one expected him to get it"

• noun:   a spy or saboteur or terrorist planted in an enemy country who lives there as a law-abiding citizen until activated by a prearranged signal
• noun:   a passenger car that has berths for sleeping
• noun:   one of the cross braces that support the rails on a railway track

  Example: "The British call a railroad tie a sleeper"

• name:  A surname (rare: 1 in 100000 families; popularity rank in the U.S.: #9631)