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Used of sleep, meaning sleep with no dreams in it — and by extension, the deep, heavy, 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 — 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.
無夢的（多半說「一夜無夢」，指睡得很沉、很安穩）
組成dream + -less. The suffix -less means 'without', and it comes from an old word for losing — the same idea sitting inside 'lose' and 'loose'. It attaches to nouns all over English: hopeless, useless, endless, homeless.
原始詞典資料1 筆記錄
以下是外部詞典記錄的用法，原文照列，我們還沒有寫成教學內容。上面的說明才是我們自己寫的；有些用法可能上面已經講過了。
dreamlessadj
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Without dreams.
- She spent the night in a deep and dreamless sleep.
- Old Yew, which graspest at the stones That name the under-lying dead, Thy fibres net the dreamless head, Thy roots are wrapt about the bones.
資料來源：英文維基詞典 English Wiktionary（CC BY-SA 4.0）。 原文照列，未經改寫。