Some people suffer more from jet lag than other people do. Symptoms may last for one day or several days. Chronobiologists, the scientists who study the effects of time on living things. say that the seriousness of your reaction depends on several factors. One factor is the number of time zones you crossed. Another factor is whether you flew east to west or west to east. It is easier to adjust after an east-to-west flight. Personality factors also affect how easily you adapt to the new time for example, "night" people adapt more easily than "morn-ing" people. Extroverts (outgoing, sociable people) adjust morn easily than introverts (shy, quiet people). Flexible people who don't mind changes have fewer problems than inflexible people who are rigid and don't like change. Younger people suffer less than older people. Healthier people get over jet more easily than people who are sick.