You may not have known this, but your body is specifically used to the times that you wake up, go to sleep, and go about your daily routines. This is called the Circadian Rhythm, or Circadian Clock.
Your body uses sensory nerves in your eyeballs to determine what time of day it is. The nerve uses light color tones to determine when it's time to wake up and when it's time to start shutting down. For example, the warmer colors of sunset remind your body that it's the evening and time to start winding down.
When you travel long distances at extremely fast rates, such as on an airplane, you might travel across time zones, where that part of the world will have a different time than you're accustomed to.
As a result, say you were travelling from an Eastern state to a Western state, you would be prepared to go to sleep earlier than you normally would because you just added more time to your Circadian Clock that throws it out of whack and makes you feel tired earlier. This is known as Jet Lag.
You can prevent Jet Lag by going to sleep earlier a few days before your trip such that when you arrive in the new time zone, you're already ready to adjust to the new time zone.