From Albert Barnes Notes on the Bible:
The Jews anciently divided the night into three parts of four hours each, usually called watches. The first of these watches is mentioned in Lam_2:19, the middle watch in Jdg_7:19, and the morning watch in Exo_14:24. In the time of our Saviour they divided the night into four watches, the fourth having been introduced by the Romans. These watches consisted of three hours each. The first commenced at six and continued until nine; the second from nine to twelve; the third from twelve to three; and the fourth from three to six. The first was called evening; the second midnight; the third ****-crowing; the fourth morning, Mar_13:35. It is probable that the term watch was given to each of these divisions from the practice of placing sentinels around the camp in time of war, or in cities, to watch or guard the camp or city; and that they were at first relieved three times in the night, but under the Romans four times. It was in the last of these watches, or between three and six in the morning, that Jesus appeared to the disciples, so that he had spent most of the night alone on the mountain in prayer.
