Do I need a visa at Beijing Capital airport if staying in the airport for 18 hours?

I want to fly with Air China from Fukuoka in Japan to the UK. My flight from Japan arrives in Beijing at 1830 and departs at 1235 the following day. My travel agent says the airport is not open 24 hours and that I will need a visa. After reading many comments online I do not think this is true. Can you tell me if it's ok for me to fly without a visa and then stay in the airport for 18 hours until my connecting flight? I plan to just read and maybe get a room at the lounge for some of the time. Thanks.