Is there a scientific basis for killing pigs in the middle of the night and selling pigs in the morning?

There is no scientific basis. This is just a way for illegal companies to reduce costs. Kill pigs in the middle of the night and sell them in the morning, so that the meat quality will not go bad even if it is not precooled. So this saves the production cost. The scientific way is to kill pigs in the morning or afternoon, then precool and drain acid for 7- 12 hours, and then enter the cold meat market.