Let me give you a concrete example to set the scene — Page 216, Lesson 346

I want to walk you through a topic that's absolutely central to your life as a professional pilot: the disruption of your body's internal rhythms when you cross time zones, and how you recover from it. We call this whole area of study "aveller" — it's the term for the study of the effects of time-zone changes on the human body, particularly in aviation. Let me give you a concrete example to set the scene. Imagine you're a pilot who has just flown from the UK to Los Angeles. It's local noon in LA — bright daylight, lunchtime for everyone around you. But your body clock, your internal circadian rhythm, is still running on UK time, which is 2000 hours — eight in the evening. Your body thinks it's time to wind down for the night, not be wide awake. That's an internal conflict: your environment says "day," but your physiology says "night." And after you do manage to get some sleep, your body clock will tell you it's time to wake up when the local time is only 0100 hours — one in the morning. You're wide awake at 1 a.m. local time because your body thinks it's a reasonable UK morning. Now, why does this matter so much to you as a pilot? Because you may have to sleep during local daytime hours, or operate a long flight at a time when your body clock is screaming at you that it's time for sleep. That's a direct threat to performance and safety. And it's not just your sleep-wake cycle that gets disrupted. The normal rhythms of your alimentary canal — that's the passage along which food passes during digestion — and your urinary system also get thrown off, and that can cause further disruption to your sleep in the new time zone. So how do you recover? The key is something called "zeitgebers." Zeitgebers are the external cues that help synchronize your internal body clock to the local environment — things like light, meal times, social activity. Shifting these zeitgebers helps you resynchronize to the new local time, but it is a slow process. On average, your body clock shifts by about 90 minutes for each day you spend in the new time zone. Let's put that into real numbers. A direct flight from London to Los Angeles involves a shift of 9 hours in local time. At 90 minutes of adjustment per day, that means it will take about 6 days for your body to fully adjust to local time. But here's the problem: as a pilot on a typical schedule, you may only have 2 or 3 days before you return to London. And when you do return, your body clock is now out of synchronisation all over again — you haven't even finished adjusting to LA before you're thrown back into UK time. There's another complicating factor I want you to understand. Different body systems shift their phase at different rates. So while they are shifting, they are not only out-of-phase with the local time, but they are also out-of-phase with each other. Your sleep-wake cycle might be adjusting at one rate, your digestive system at another, your hormone cycles at yet another. This internal desynchronisation between your own systems adds another layer of fatigue and discomfort. Finally, the direction of travel matters enormously. The effects of jet lag and your recovery from it depend on whether you're flying east or west. We'll look at specific examples of that in a moment.