Where does our sense of time passing come from? Besides breathing and heartbeat, does the human body have another source of internal rhythm? As this program illustrates, scientists have discovered a stopwatch within the brain that regulates body chemistry and gives us our awareness of time. Host Michio Kaku interviews the geologist who first theorized the existence of the internal clock while in subterranean isolation; monitors an experiment that reveals the time distortion experienced during trauma; meets a father and daughter who, due to a rare genetic disorder, have body clocks that run too fast; and visits a captive gorilla with a rudimentary sense of time. Original BBC broadcast title: Daytime. (50 minutes)