It's not easy to adjust to a new time zone or work schedule. Our body has a natural sleep/wake cycle and disruptions to it can lead to more than just feeling tired or exhausted.
Washington University professors Paul Gray and Erik Herzog are studying the biology behind our daily internal clock, or circadian rhythm. Sometimes referred to as the body or biological clock, Herzog defined it as "the entity within the body that synchronizes with an environmental cycle." This is not to be confused with the biological clock some refer to when thinking of a woman's desire to conceive.