Sleep is a condition in which I refuse to have anything to do with the outer world and have withdrawn my interest from it… we withdraw again periodically into the condition prior to our entrance into the world: that is to say, into intra-uterine existence.
Freud defines sleeping as a psychological withdrawal of interest from the external world constituting a periodic regression to intra-uterine conditions, establishing the foundational psychoanalytic concept of sleep as libidinal retreat.
, Introductory Lectures on Psycho-Analysis, 1917thesis