Final answer:
The most suitable word to complete the sentence is 'enigma,' as it denotes something deeply puzzling or inscrutable that cannot be overcome by sleep or time.
Step-by-step explanation:
The sentence in question is looking for a noun that fits the context of something that is perplexing and cannot be easily understood or overcome by sleep or time. In this context, the best option among the provided choices would be enigma, because it refers to something that is mysterious, puzzling, or difficult to understand. While "mystery," "riddle," and "conundrum" are similar in meaning, an enigma implies a deeper level of mystery that is often unsolvable, which fits the description in the sentence because it cannot be mastered by sleep or time.