The middle watch. — The Jews anciently divided the night, from 6 P.M. to 6 A.M., into three watches (Exodus 14:24; 1 Samuel 11:11). The subsequent 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 — i.e., soon after 10 at night — would be the time at which the host 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.