I've heard about adepts getting their sleep shortened or completely replaced by deep meditation but I've never actually found such person! I'd like to. One acquaintant of mine who is a teacher in chan buddhist tradition comes closest. He said he sat all nighters for 1-1½ year in his training days but then returned to 6 hours of sleep. There is another Zen-teacher I know who says he never looses awareness state, not even in sleep, not even for a moment but he didn't convince me as he doesn't appear advanced.
To me its mind boggling that anyone serious about liberation would willingly go to unconscious rest night after night. It is a fact that we need to have our rest (and it's good that it can be enjoyed!) if our state hasn't ripened but still I've never found in-depth explanations and commentaries about emptiness meditation during sleep. There is a page here and there but nothing comprehensive. Does anyone know if such a document exists? Has any teacher or master ever explained this stuff in detail? How many practitioners have ever achieved such a state? 1? 2? More?
In my experience falling asleep has 5 steps. These steps lead to deep sleep without dreams or images which is sometimes called causal body. That is where the roots of karma is stored. To be able to remain aware of this field of experience in sitting, I'd say can be reasonably easily attained but in supine position it is a great challenge. Stream entry is piece of cake compared to this one. Perhaps such persons do exist somewhere but I've become quite amazed that there is so little info about this. You know, it doesn't make sense that there are roshis, rinpoches, swamis and whatever everywhere on this globe but no one has covered this issue. Are they liberated only during day time?