Plants do the opposite of photosynthesize during their photo dark period. They transport chemicals and enzymes throughout the night repairing and preparing for tomorrow just like humans do.
Real world growth rates do not support the 24-hour light theory.
Going from 18 to 24 hours of light is a whopping 33% increase in watt hours which should translate into 33% bigger plants, but it doesn’t. It only translates into 33% more electricity being spent.