Used of sleep, meaning sleep with no dreams in it — and by extension, the deep, heavy, restful kind. That extension is the whole point of the word. Strictly it only tells you what was absent, but nobody says 'dreamless' to report a fact about dream activity; they say it to mean the sleep went down so deep that nothing surfaced, and you woke feeling like no time had passed. So it is a compliment about rest. It appears almost only in the phrase 'a dreamless sleep', and it is bookish — you meet it in novels and in careful writing, while in conversation people say 'I slept like a log' or 'I was out cold'. One thing worth knowing: in older literature 'dreamless sleep' is sometimes a gentle way of talking about death, so if the surrounding sentence sounds solemn, check which one is meant.