9 But why at the sixth hour? Because at the sixth age of the world. In the Gospel, count up as an hour each, the first age from Adam to Noah; the second, from Noah to Abraham; the third, from Abraham to David; the fourth, from David to the removing to Babylon; the fifth, from the removing to Babylon to the baptism of John: thence is the sixth being enacted. Why do you marvel? Jesus came, and, by humbling Himself, came to a well. He came wearied, because He carried weak flesh. At the sixth hour, because in the sixth age of the world. To a well, because to the depth of this our habitation. For which reason it is said in the psalm: “From the depth have I cried unto You, O Lord.” He sat, as I said, because He was humbled.
Source: Tractates on the Gospel of John (New Advent)