When I was younger, I used to take the viewpoint that dreamless sleep was a window into our own oblivion. I took it as self-evident that sleep represented an absence of consciousness - but have since discovered the issue is far murkier than I suspected. Hypnagogic states, amnesia during sleep onset and brief awakenings during the night, and other phenomena show a great deal of cognition occurs even in deep sleep. Whereas these phenomena fall short of concluding an apathetic and/or forgetful state of self-awareness, they certainly prevent it from being dismissed out of hand.
I'm curious what others here think on the subject. I find both the possibilities - sleep as a preview of death versus an "alien" state of consciousness - intriguing both philosphically and artistically. I'm also interested in any fiction that might treat this theme specifically - or dreamless sleep in general. My favorite story by Lovecraft is the much overlooked "Hypnos". I'm unaware if Ligotti has touched on the theme - but so far I've only read about half of his writing...
For those interesting, I've also found a decent philosophical paper on the subject freely available:
Henry Johnstone also wrote two heavily cited papers on the subject in the 1970's - "Towards a Philosophy of Sleep" and "The Phenomenology of Death" - although I have not been able to read these due to pay walls.