After two full years in prison, Pharaoh dreamed (Genesis 41:1). The midrash reads this through Psalm 73: "As an endless dream, the Lord despised their form." God does not reveal Himself to the wicked during daylight — the encounter would shame them, like a dream dissolving in morning light. The wicked operate in obscurity, in the darkness where their deeds can remain hidden. So God meets them at night, in the dream register, where their power is already diminished.
This is also why Jacob's famous ladder dream came to him at night. The ladder, the angels ascending and descending, the divine voice at the top — none of it came in daylight when Jacob could have doubted his senses. It came in sleep, in the vulnerable space where defenses are down and the soul is open. The rabbis taught that prophecy came to the prophets in the night watches precisely because night removed the daytime noise of self-importance and self-sufficiency.
Pharaoh's dreams are different — not revelation but signal, requiring a human interpreter to decode their meaning. The fat cows devoured by the thin, the full grain consumed by the withered — these are warnings, not promises. God sends warnings to those who might act on them, even to Pharaoh, because the seven years of plenty and famine belong to Egypt's history and not only Israel's. Joseph in prison, forgotten by the butler for two full years, was precisely where God needed him to be when Pharaoh's dream required someone who knew how to receive divine communication.