NATI HARNIK—AP

Michael Phelps, 23, Swimming

The biggest challenge Michael Phelps faces in Beijing will be the clock. And not the one at the pool. With all of the final races in swimming moving from their traditional evening time slots to refreshing dips first thing in the morning, Phelps' thorniest adversary will be the alarm clock. "I am far, far from being a morning person," he admits. "If I have a workout at 7:30 a.m., I'm hitting the snooze button at 6:50." Still, even the rise-and-shine races aren't likely to slow Phelps down. Since the Athens Games in 2004, he has set six new world records, in most cases besting his own history-making times. In Beijing, he is again scheduled to swim up to eight events, giving him another chance to surpass Mark Spitz's record of seven gold medals at a single Games. As long as Phelps wakes up on time, the race for gold could be over early.

--By Alice Park