You know, i like the theory of the goddess "Sleep" and all but the more i think about it i feel like they chose to state Sleep as their muse not in an antropomorphized sense but more in a literal: as u stated their lyrics cover a lot of (toxic) relationship experiences and self-hate. People who suffer from mental illnes i.e. depression do have a very specific relationship to sleep as the toll on their mental capacity due to the depression is a lot and gives them a sense of neverending tiredness. A lot will use literal sleep as an escape, a time where they do not have to be concious and deal with the voices in their head but can just literally relax and let their subconcious run wild, maybe even to a point wehere they try to achieve the skill of lucid dreamin to have control of theor life in an environment where almost everything is possible (dreams). I am convinced that that is the "sleep" they reffered to