The physiological and mental stress encountered by airplane travelers when crossing four or more time zones is commonly called jet lag. Patterns of hunger, sleep, and elimination, along with alertness, memory, and normal judgment, may all be affected. More than 100 biological functions that fluctuate during the 24-hour cycle (circadian rhythm) can become desynchronized. Most people’s bodies adjust at a rate of about one hour per day. Thus after four time zone changes, the body will require about four days to return to its usual rhythms. Flying eastward is often more difficult than flying westward, which adds hours to the day.