Almost everyone who has traveled across more than a few times zones in one trip has experienced the debilitating effects of jet lag. While travel across vast distances is now rapid, convenient, and commonplace, we are still saddled with biological limitations arising from millions of years of evolution. The distress associated with jet lag results when the body's internal clock, or circadian rhythm, becomes desynchronized with the external time zone.
Jet lag is characterized by unpleasant symptoms, including insomnia, sleepiness, impaired performance, diminished alertness, irritability, depressed mood, and gastrointestinal distress (Waterhouse 2005). Symptoms of jet lag are slightly more dramatic for travelers heading east. In addition, older individuals are likely to suffer more from its effects (Monk 2005).
The human circadian rhythm (characterized by rising and falling hormone levels, undulating body temperature, and the familiar sleep-wake cycle) is linked to the rising and setting of the sun. Through its production of melatonin (ie, the circadian hormone), the pineal gland plays a crucial role in circadian rhythm.
Research suggests that the jet aircraft environment itself may also contribute to jet lag. In a recent experiment, researchers simulated the mild oxygen deprivation (hypoxia) that occurs in pressurized aircraft cabins during long-duration flights at altitudes between 8000 and 12 000 feet. Participants were assessed for changes in melatonin levels. Scientists found a significant decrease in nightly peak of melatonin, prompting speculation that hypoxia induced by cabin air contributes to post-flight fatigue after long flights and to the clinical disorder of jet lag (Coste 2004).