What does the cryptic crossword clue "How we might sleep some day? Just the opposite (3,5)" mean?

The wordplay uses a charade (building blocks). ALL (synonym="ALL") + NIGHT (synonym="NIGHT") = ALLNIGHT The answer is ALLNIGHT.

How we might sleep some day? Just the opposite (3,5)

Guardian Cryptic #29240 | 17 Down | 2023-11-29 flower