Midrash su Salmi 76:8

What does bakosharot (“prosperity”) imply? The one who is not pleased (with his mate) weeps, the one who is pleased sings. Hence the word bakosharot (from beki, “weeping,” and shirot, “songs”) is used in the text. He causes one to ascend the ladder and another to descend. Thus it is said: For God is judge; He putteth down this one, and lifteth up this one (Ps. 76:8).

Another explanation: "And it was in the middle of the night" - there is a parable of a king, the country of which overturned the subjugation of prisoners that came to them [by making it unbearable]; so [too] did he overturn the law against them and kill them. Likewise, the Egyptians overturned the decree against the Jews, that they should [work] by day and by night and that they gave the work of a man to a woman and the work of a woman to a man; so [too], God overturned the law against them and killed them at night, as it is stated, "And it was in the middle of the night." Just like He overturned Sodom at night, so [too] did He kill the firstborn of Egypt at night. For this reason did David state (Psalms 76:8), "You are Fearsome, You; and who can stand before You from when You become angry."