How are some of Jet lag symptoms attributable to the observed changes in cortisol and TSH?
Jet lag is a well-known sleep disorder arising from crossing multiple time zones in a short period. However, it probably has underlying endocrine pathophysiology. Essentially, the circadian process becomes misaligned with the destination time zone. This is particularly true when flying east, which moves time zones ahead and puts the individual at risk for insomnia, whereas flying west moves the time zones back, putting the traveler at risk for daytime sleepiness in the new time zone. For example, if a physician departs London on a nonstop 10-hour flight west to Los Angeles, the doctor’s circadian clock is set to London although his or her body is now located in Los Angeles. London is 8 hours ahead of Los Angeles. So, if the flight leaves London at 8 am, he or she will arrive in Los Angeles at 6 pm London time (in his or her circadian clock); but because of the 8-hour time difference, it is 10 am Pacific Standard Time (PST). He or she clears customs and, by noon, is home exhausted, sleeps 8 hours, and wakes at 8 pm PST. You can see that this person will struggle with unwanted sleepiness and insomnia until his or her clock shifts from London time to PST.
Patients with insomnia, whose total time asleep/total time in bed is < 70% of normal, have significantly higher evening and early sleep cortisol levels. In a study of young adults whose circadian rhythms were perturbed by a flight from Europe to the US, GH secretory patterns adjusted within a few days to the new sleep–wake cycle, but the cortisol levels remained disassociated for 2 weeks . This dissociation is thought to contribute to the symptoms of jet lag syndrome. Disruption of the hypothalamic–pituitary–thyroid axis during prolonged flight has also been studied. The usual TSH-suppressive influence of sleep may not be present in prolonged air travel because the traveler remains awake. This translates to an overall TSH elevation, paralleled by a small, temporary increase in serum T 3 levels. The study related the fatigue and discomfort of jet lag syndrome to the prolonged elevation of thyroid hormone as well as to the desynchronization of multiple circadian rhythms.