Adam Clarke on Jeremiah 51:39

In their heat I will make their feasts - It was on the night of a feast day, while their hearts were heated with wine and revelry, that Babylon was taken; see Dan 5:1-3. This feast was held in honor of the goddess Sheshach, (or perhaps of Bel), who is mentioned, Jer 51:41, as being taken with her worshippers. As it was in the night the city was taken, many had retired to rest, and never awoke; slain in their beds, they slept a perpetual sleep.