A sleeper is a person who is experiencing a shared dream. Except in Limbo, one or two sleepers must take on the roles of the subject, who populates the dream with projections, and the dreamer, who constructs the dream.
A sleeper may be both the dreamer and the subject, even if others are present in the dream. Other sleepers can manipulate reality in the dream but not the fundamental construction of the dream. A sleeper's body within a dream is a projection of their mind and can thus be manipulated by a sleeper if they know how.