Russel has extreme insomnia due to his ability to sense and hear spirits around him. the constant presence of ghosts and the memories of his past  keep him up at night. It was especially bad in phase 3, when he felt that Murdoc and 2D had abandoned him. Now that he’s back in phase 4, he’s starting to get more and more sleep (if not still being bothered by the demons in the spirit house from time to time).