A rhythm is something that is regularly repeated. All living organisms experience rhythmic changes which tend to coincide with seasonal or daily environmental changes. Most organisms have an internal biological clock called endogenous pacemakers, which are influences by external environmental factors called exogenous zeitgebers, these control periodic changes. The sleep wake cycle is a circadian rhythm that repeats itself every 24 hours. However these can become desynchronised, resulting in a disruption of your biological rhythm. It can be disrupted by many factors, including Jet lag and shift work.
Jetlag occurs when flying east to west or vice versa, it does not occur when flying north to south or south to north because you do not pass through any differing time zones. Places to the east are ahead of the time, and places to the west are behind. Klein, Wegman and Hunt suggested that adjustments in sleep were faster for westbound flights compared to eastbound, as when you go eastbound readjustment takes approximately a day for every time zone you have crossed. This is because when travelling west you are ‘chasing the sun’ and the day is lengthened (phase delay)- the body prefers this. But when you travel east the day is shortened (phase advance) this shortens the body for what is already a shortened day, making you try to sleep when you are not tired. The disruption of the biological rhythms are thought to be caused by melatonin release being out of step with the new environmental conditions, this can cause drowsiness during the day, and insomnia during the night.
Research evidence by Schwartz et al supports this theory. They studied the results of baseball games involving teams on the west and east coast of America, the time difference was 3 hours. They found that the east coast teams travelling west won significantly more games than the west coast teams travelling east, due to the day