Jet lag occurs when a person's internal circadian rhythm is out of sync with the local time of their destination. Flight crews and passengers experience fatigue, insomnia, and cognitive sluggishness because their bodies are still releasing sleep hormones based on their departure time zone. The severity usually increases with the number of time zones crossed and is often worse when traveling eastward.
Heard on the job
I'm bidding for north-south routes next month because the constant jet lag from the Asia flights is destroying my sleep schedule.