Why is Jet Lag called Jet Lag?

It's 4am and I've officially been in Brisvegas for 22 hours now - I've slept for about 4 hours; talked on the phone to friends for about 11/2 hours; indulged myself in beauty treatments for 1 hour and 10 mins; hung out with mum at the local shops for 40 minutes; helped dad fix the pool motor for 30 mins; caught up with Aunty Marg & Uncle Peter for an hour; chilled at home with Dunk & Tash & Mum & Dad for the evening and spent pretty much every other second simply staring at Baby Isla !!!!
I know I am completely biased, but she is soooooo adorable it's not funny. Every sound, every breath, every move, is abolutely amazing...