Average transit time for food with a j-pouch = 6 to 8 hours. Breakfast at 7:00 am... comes out around 3 in the afternoon. Lunch at 12:00 pm, comes out around 6-8 pm. Dinner at 7:00 pm, comes out either middle of the night or when you get up in the morning.
The morning and early afternoon seem nice because you don't have anything moving through from the time you get up until about 3:00 pm.
That's always been my explanation.