💡 Detailed Explanation
The adverb 'そろそろ' (sorosoro) means 'soon' or 'it is about time to'. In this sentence, it indicates that the speaker should go to bed soon because they have an early start tomorrow. The other options do not fit the context: 'ますます' means 'more and more', 'だんだん' means 'gradually', and 'なかなか' means 'quite' or 'not easily' (with a negative verb).