US citizen, staying overnight in Shanghai (20 hours)

My husband and I, US citizens, are traveling to Japan from the U.S. and are staying overnight in Shanghai, China (for 20 hours - Flight arrives 5:30pm, leaves 1:30pm next day). We understand that we do not need a visa, but what is this "transit permit" ---do we have to get it? Since it is overnight, of course we leave the airport to spend the night a hotel somewhere in Shanghai. Thanks for the help....