Do I need a temporary visa stay permit? (Jessica from San Francisco)

Hi, this my first time traveling overseas from San Francisco, United States to Shanghai Pudong; where I will have a 5 hour layover before I depart into Tokyo, Japan. I will be in Japan for 10 days. Then I will fly from Tokyo, Japan to Shanghai Pudong where I will have 3 hour layover an then fly back to San Francisco, United States. Just inquiring if I will need an entry permit to get into Japan an another one to get back into the United States? I'm American do I qualify for 24-72hour free transit during this time? Or Am I required to apply for a temporary stay permit?