Charles Ellicott Commentary Judges 7:19

"So Gideon, and the hundred men that were with him, came unto the outermost part of the camp in the beginning of the middle watch, when they had but newly set the watch: and they blew the trumpets, and brake in pieces the pitchers that were in their hands." — Judges 7:19 (ASV)

The middle watch. — In ancient times, the Jews divided the night, from 6 P.M. to 6 A.M., into three watches (Exodus 14:24; 1 Samuel 11:11). The later division into four watches of three hours each was borrowed from the Romans (Matthew 14:25; Mark 6:48). At the beginning of the middle watch — that is, soon after 10 at night — would be the time at which the army would be buried in their first sleep.

They had but newly set the watch. — Literally, scarcely — or, “just in rousing they roused the watch.” The attack took place at the moment of confusion caused by changing the watch.