Jet lag is the tiredness you may feel after flying long distances across several time zones. The rapid travel to a different time zone disturbs your normal body rhythms.
How does it occur?
Your body has an internal clock that determines when you eat, sleep, and wake up each 24-hour day. When you travel across several time zones, your "day" is longer or shorter than 24 hours. Your body is out of sync with the local time zone. It takes some time for your body rhythms to adjust to this shorter or longer day. While you are adjusting, you may feel jet lag.