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Jet lag is primarily caused by disruption of the body’s circadian clock rather than simple sleep deprivation. It can produce fatigue, digestive problems, headaches, impaired concentration and memory, and—in cases of repeated disruption among airline crews—may be associated with hormonal, neurological and longer-term health risks, although much of the evidence comes from observational or animal studies. Symptoms tend to worsen when crossing more time zones, traveling eastward and experiencing shorter days. Helpful measures include gradually shifting sleep times before travel, aligning meals and activity with the destination’s schedule, carefully timed exposure to light and light exercise, and allowing sufficient rest. Melatonin may help some travelers but has uncertain effectiveness, timing and safety, so medical advice is recommended; controlled light exposure is presented as a safer option. Most people adapt gradually, usually improving within about a week.