Answer: Anyone who has spent time in a juvenile prison
During the course of the film, the adult Shakes, (played by Jason Patric), reveals that the term "sleepers" is "a street name for anyone who" has "spent time in a juvenile facility", which includes the four friends, Michael, Shakes, Tommy and John. The film is based on the 1995 novel of the same name, written by Lorenzo Carcaterra, and both open with the same lines: "This is a true story about friendship that runs deeper than blood". Whether or not there is any real 'truth' in the story has been the subject of much debate and controversy, but either way, the film is worth watching, and is powerful in its own right, no matter what the "truth" of its story is.