Have you ever taken a long-haul flight? If so, you know that the timepiece in your head sometimes ignores the one on your wrist. If you leave Boston in the evening and, seven hours later, arrive in Paris at breakfast time, your body screams, "Why am I getting up? It's the middle of the night!" Croissants or no, your internal clock persists in its own rhythm, and it can take several days to synchronize your sleeping patterns with your new surroundings.