Circadian rhythm. Circadian rhythm are physical, mental and behavioral changes that follow a 24 hr cycle. They respond to light and dark in an environment. Travel across time zones will often upset our body's natural clock making it out of sync with the destination time. Rhythms dictating time for eat, sleep, hormone regulation and body temp. No longer correspond to one's new environment.
Time change. Time change and when it gets lights and dark affect your circadian rhythm. Body is responding to light and dark cues so when people travel those clues are given at "incorrect" time. Your body doesnot kow what to expect until it adapts over a few days. In general expect about 1-2 days for every hour of time difference.