Sleep and Fatigue — Page 216, Lesson 345

I want to walk you through the topic of time zone crossing and its effects on aircrew. Let's start with the general picture. Crossing time zones is a way of life for long haul aircrew. Every time you cross a time zone, your body's internal clock — your circadian rhythm — is shifted away from the local time at your destination. These shifts can lead to cumulative sleep deprivation. That means the sleep debt builds up over several flights or days. However, the text notes that although such sleep deficits can build up, it is unlikely to go to extreme levels because the body will sleep when it needs to. Your body has a powerful drive to get rest, even if the timing is wrong. Long haul pilots have constantly to adjust and readjust their circadian rhythms. This repeated disruption is not without cost. The text says it is possible that continual disruption may incur some health penalties, particularly associated with stomach and bowel disorders. So we're talking about digestive system problems as a known risk of chronic circadian disruption. The disturbance to the normal body functions is commonly known as jet lag or, using the more precise medical term, circadian dysrhythmia. Let me define that properly. Circadian dysrhythmia, also called transmeridian desynchronisation, is the condition where your internal body rhythms become out of sync with the local time at your destination. The internal body rhythms become of great significance in the modern age of rapid air travel. This leads to a large discrepancy between the local time at destination and the body clock of the traveller. Let me give you the example from the text. Imagine you are a pilot flying from the UK to Los Angeles. It may be local noon for the traveller arriving in Los Angeles — that's 12:00 PM local time, bright daylight. But his body clock will still be based on a UK time. So if the UK time is, say, 8:00 PM, his body thinks it's evening and wants to wind down for sleep, but the local environment says it's midday. That discrepancy — that mismatch — is the core of circadian dysrhythmia. Now, before we move into the next part, I want to connect this back to the shift rotation guidance we just covered. The text stated that generally it is now accepted that shift rotation should be to later shifts — early shift to late shift to night shift and so on. That principle of rotating forward applies to time zone crossing as well. When you travel westward, you are effectively moving to a later time zone, which is easier for the body to adapt to than travelling eastward, which moves you to an earlier time. But we'll get into the specifics of that as we continue. For now, the key takeaway is this: time zone crossing causes a mismatch between your body clock and local time. That mismatch is called circadian dysrhythmia or jet lag. It leads to cumulative sleep deprivation, and over a career, the continual disruption can carry health penalties, especially stomach and bowel disorders. Your body will eventually force sleep when it needs it, but the timing and quality of that sleep are compromised.