A journey across multiple time zones the body's rhythms at first retain the characteristics of their point of departure. However, the new environment forces new influences on these cycles, the main factors being the time of sunrise and onset of darkness.The body attempts to adjust to this new context but core temperature is relatively sluggish in doing so. As a rough guide it takes about one day for each time zone crossed for body temperature to adapt completely.
The individual may have difficulty in sleeping for a few days, but activity and social contact during the day help in accelerating the adaptations of the arousal rhythm. Thus arousal adjusts more quickly than does body temperature to the new time zone. Until the whole spectrum of biological rhythms adjusts to the new local time, the performance of exercise may be below par