Jet lag is caused by the disruption of the body clock as it is out of synchronisation with the unfamiliar time zone of the destination. The body experiences different patterns of light and dark than it is normally used to which disrupts the natural sleep-wake cycle.
Melatonin is a hormone that plays a key role in body rhythms and jet lag. After the sun sets, the eyes perceive darkness and alert the hypothalamus to begin releasing melatonin, which promotes sleep. Conversely, when the eyes perceive sunlight, they tell the hypothalamus to with hold melatonin production. However, the hypothalamus cannot readjust its schedule instantly, it takes several days.