"And yet i would not sleep; merciful powers, restrain in me the cursèd thoughts that nature gives way to in repose" (2.1.7-9). This is Banquo complaining about his inability to sleep as a result of nightmares. This picture is an open eye, representing the inability to sleep because of what's inside the mind (depicted in the person within in the eye in this photo)
As I slumber Id and Ego must be bound. For if they were free, my nightmare would get out and skulk around. It could grotesque or surprisingly mild. Either way it tries to grab and devour my Inner Child. Why they do this I just don't know. Perhaps it would not happen if I possessed an older Soul.