The warlord's alarm

Chuan and Jing Jing use their mathematical skills to ensure that the warlord wakes up on time. They are traveling to the emperor's palace, and they can only sleep for four hours before resuming their journey in order to arrive as the emperor's gates open. How will they wake up? There are no clocks at the inn where they are staying, so they must create their own alarm clock. Chuan and Jing Jing create a simplified Chinese water clock and awaken in time to rouse the warlord.