Symptoms may last for one day or several and vary greatly in severity. Chronobiologists, 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. Your jet lag will probably be worse if you crossed several time zones. 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 "morning" people. Extroverts adjust more easily than introverts. Flexible people who don't mind changes have fewer problems than inflexible people who are rigid and don't like changes. Younger people suffer less than older people. Finally, healthier people usually get over jet lag more easily than people who are sick.