The night split for sentry duty — three watches in the Old Testament, four in the New, because Rome counted differently.
Night was divided by whoever was standing guard. The Hebrew reckoning is three watches — beginning, middle and morning — while the Roman army used four, and the gospels follow Rome. Mark names all four in one sentence when he says nobody knows when the master comes: evening, midnight, cockcrow, or morning. So "the fourth watch of the night", when Jesus comes walking on the water, is somewhere between three and six in the morning — the last and worst shift.
Drawn at an equinox, when the sun rises about six. An hour was a twelfth of the daylight, so summer hours were longer than winter ones and the clock times are only ever approximate. Night was split by whoever stood guard: three watches in the Hebrew reckoning, four in the Roman one the gospels follow.