Jet lag symptoms tend to be more severe when the person travels from westward compared to eastward.
Our circadian rhythms are driven by an internal time-keeping system. This biological clock is entrained by external environmental occurrences, such as the light-dark cycle of night and day. Put simply, our circadian rhythm regulates our daily activities, such as sleep, waking, eating and body temperature regulation. Problems readjusting our internal biological clock causes jet lag, as do problems with shift work, and some sleeping disorders.