Jet lag, also called jet lag disorder, is a temporary sleep problem that can affect anyone who quickly travels across multiple time zones. Your body has its own internal clock (circadian rhythms) that signals your body when to stay awake and when to sleep.
 
Jet lag can occur anytime you cross two or more time zones. Jet lag occurs because crossing multiple time zones puts your internal clock (circadian rhythms), which regulates your sleep-wake cycle, out of sync with the time in your new locale.
 
Bright light exposure is the most powerful way to cause a phase shift — an advance or delay in circadian rhythms. Light in the early morning makes you wake up earlier (“phase advance”); light around bed time makes you wake up later (“phase delay”). ... Thus, seeking and avoiding light at the right times can reduce jet lag.