I honestly think it Really doesn’t matter but I would say right when lights go on. Transplants can go pretty smooth if your real gentle with it Light on/off it’s gunna stall for a bit anyway.
outside in the veggie garden i like to transplant late in the day, so they have a chance to settle in before the hot part of the next day.! indoors you don't have as much heat or light intensity so transplanting should be done at a convenient time so you can take your time and transplant carefully with less stress to the plant.! after a transplant simply place the light a little higher for a day or 2 then lower it to the proper height. good luck Mike.!!