Falling into a Deep Sleep

I've been contemplating why we refer to the unconscious as if it were an abyss or a deep well mirroring in its bottom "the face of the deep," the dim reflection of the unconscious Ego. Why do we call a sleep "deep," and why do we "fall" asleep, as if one were submerging in a bottomless gulf? "Darkness like a dream," or "darkness of death," Shakespeare says.

I don't know....