According to Alan Burdick, author of Why Time Flies – A Mostly Scientific Investigation, “The body recovers at a rate of about one time zone per day. Jet lag is not in your head; it's an ailment of your entire, desynchronized body.”

So you're not lazy or out of shape or a bad traveler! Well, actually, you may be all of those things. And they may contribute to your feelings. But they aren't solely responsible for your jet lag. It's a very real condition.

Here's the except from Burdick's book that explains what's actually happening when we get jet lag:

How the brain's suprachiasmatic nucleus disseminates its time throughout the human body is still poorly understood. But the process takes time–hours to days. If you're subjected to a sudden shift in your light regime and are forced to adjust to a new schedule–the sort of thing that happens when you cross a few time zones, or even for the day or two following the switch to or from daylight savings time–your peripheral clocks don't fall back in line all at once or at the same rate. Your body ceases to be a synchronized confederacy of clocks and instead becomes, temporarily, a conflagration of temporally autonomous states. That's the essence of jet lag. When my suprachiasmatic nucleus lands in New York, my liver may still be on Nova Scotia time and my pancreas may be somewhere over Iceland. For a few days, my digestive system will be out of whack, as my brain directs me to eat food at hours when my organs aren't fully aligned to metabolize it. (The body recovers at a rate of about one time zone per day.) The result is gastroenteritis, a common complaint of long-distance travelers and airline pilots. Jet lag is not in your head; it's an ailment of your entire, desynchronized body.

So have compassion for your friend who just home from a trip across the world and who you haven't seen in weeks. They want to see you too, but probably need a little time to catch up before meeting you for a drink at the bar. Who wants to drink alcohol when your liver is still hours away?