“fourth watch of the night.” At the time of Christ, both the Jews and Romans divided the night into four watches, each being three hours long: 6-9 PM, 9-12 PM, 12-3 AM, and 3-6 AM. So the fourth watch of the night started at our 3 AM (see commentary on Mark 6:48 and 13:35). So this is sometime between 3 AM and 6 AM. Since the Apostles had started sometime before sunset (Mark 6:47) and it was now at least 3 AM, so the Apostles had been rowing for at least six hours to go about 5 or 6 miles.