The processes of all animals and plants follow a 24-hour rhythm called body clock. This is our personal synchronization with the daylight-darkness cycle. When you travel in an airplane crossing time zones, say from New York to Houston, your body clock cannot adjust itself quickly enough. Your will lose sync with the new daylight-darkness cycle of where you left (NY) and does not recover quickly for the place that you are going (Houston). This causes a condition of disorientation called jet lag.