This handout shows what is meant by the sixth hour of the day or the watches of the night as mentioned in the New Testament.
The Roman clock, or time of day, was divided into 12 hours (Latin horae) of light and 12 hours of darkness.
Since the length of the sunlight varied with the seasons this also meant that the length of the hour changed – with shorter hours in winter and longer hours in summer. Hours could vary in length from 45 minutes to 75 minutes.
The Romans divided the night into four watches, following the Greek practice, since, as Vegetius explains, a city-guard could not stand watch all night. For example, “in the fourth watch of night” (quarta vigilia noctis) meant just before dawn.
See Wikipedia, Roman timekeeping system.