during the day as it is more natural to have heat during the day and cooler during the night.
plus during the day there should be light so the snake can come out and bask, during the night it should be dark.
even though we can not provide exactly the same natural enviroment as they have in the wild, its best we try and keep things as close to natural as we can.
I have a thermostat that allows me to set a warmer daytime temp and a slightly cooler night time temp. I reduce these slightly in the cooler months and my Jungle uses the whole enclosure and chooses the temp that he requires.
He has both warm and cool places to both sunbake (for lack of a better word!) and hide. It's important to give them choices.