What is happening to your body when you have jet lag?

Circadian : Your inner clock is off...It takes several days for your circadian rhythm to catch up..

Jet lag: Your sleep/wake cycle in out of kilter with time zone you just entered. Jet lag symptoms: Fatigue or sleepiness, decreased concentration, moodiness, GI changes (diarrhea, nausea or constipation), headache, sense of not being well & sleep disturbance (insomnia or early morning waking).