I wondered about this too. Best evidence I can find is the weather in my area. Most of our rainfall occurs during the daylight hours. Even accounting for there being more daylight hours than non-daylight hours the vast majority of my rainfall is during the day. Based on that tidbit I water my flowering room an hour or two after the light comes on at 8 AM. I harvest a perpetual grow and have noticed that plants seem to feel stickiest a few hours after the daily watering.
Good luck, BigSteve.