"Sleepless" marked the first on screen appearance of Steven Williams as Fox Mulder's new source, "X." The character was originally intended to be a woman, and an actress was cast in the role, but after shooting her first scene the role was recast with Williams, an actor who had previous experience with writers Glen Morgan and James Wong. Natalija Nogulich, the actor who originally earned the role, was replaced because the writing staff felt she was not able to create the "right chemistry" between her co-stars.