I think it's saying that if we don't sleep we'll turn into these mad evil human-esque creatures. By sleeping we stop these creatures from taking over our bodies and escaping into the world.
So basically it's meant to scare you into sleeping rather than stop you from sleeping.
Perhaps it's meant to imply that it's the reason we sleep? We don't actually need to sleep, but we do it to keep that side of us repressed and through generations of fear of it we've changed into thinking that sleep is natural and required, whilst forgetting about the real reason? That might be thinking about it too much though..
They say that we are all the same with the subjects. Everybody is an animal inside and we avoid admitting it and sleep to repress our wildness.Or something like that....
