Long-haul travelers have long complained about jet lag’s grogginess and irritability. But researchers say the condition goes far deeper than fatigue, disrupting cell repair, altering liver and kidney function, and, in cases of chronic exposure, potentially raising the risk of cancer.
A Disrupted Internal Clock
The root cause of jet lag is not simply lost sleep, scientists say, but disruption to the body’s internal clock, which runs on a roughly 24-hour cycle governing when people eat, sleep and stay active.
Under normal conditions, light-sensitive cells in the eye detect darkness and trigger the release of melatonin, the hormone that induces sleep. That process is coordinated by a region of the brain known as the suprachiasmatic nucleus, or SCN.
Russell Foster, a professor of circadian neuroscience at the University of Oxford and author of the book “Life Time,” compared the SCN to an orchestra conductor that keeps the body in sync with cycles of night and day. Long-haul flights, he said, throw that system into disarray.
Symptoms tend to worsen with the number of time zones crossed, according to researchers, and are typically more severe on eastbound flights and during shorter winter days.
Airline Crews Face Chronic Exposure
While most travelers experience jet lag only occasionally, airline crews face it as a routine part of their jobs. Research has found that flight attendants working demanding schedules with little rest between flights had higher levels of the stress hormone cortisol than colleagues with lighter workloads.
Elevated cortisol helps the body manage short-term stress, but sustained exposure has been linked to a weakened immune system and increased cardiovascular risk, researchers say.
Brain scans of the flight attendants with heavier schedules also showed smaller right temporal lobes on average, and that group scored lower on memory tests, according to the research.
A separate study at the University of California, Berkeley, simulated jet lag in hamsters over four weeks, mimicking a schedule of transatlantic flights twice weekly. The animals struggled with tasks they normally handled easily, and researchers found they had stopped producing new brain cells in the hippocampus, the region tied to memory. The effects still had not reversed a month later.
Possible Links to Cancer
Laboratory studies have also raised concerns about tumor growth. Mice with induced jet lag showed faster tumor growth, while restoring a normal circadian rhythm slowed tumor development, according to the research. Separate mouse studies found chronic jet lag triggered a progression of liver disease, from fatty liver disease to hepatitis, scarring and eventually liver cancer.
Human data, while observational, points in a similar direction. A study from the 1990s found Canadian pilots had higher-than-average rates of prostate cancer. A more recent meta-analysis found female flight attendants had elevated rates of breast cancer and melanoma, and one survey found flight attendants overall reported higher cancer rates than the general population among women, though not among men.
Researchers caution that the studies are observational and cannot establish that jet lag alone drives the increased risk, since exposure to cosmic radiation during flight and other factors may also play a role.
Steps Travelers Can Take
Health experts recommend several strategies to ease jet lag’s effects:
- Adjust before departure. Travelers heading east should go to bed one to two hours earlier than usual for several days before the flight; those heading west should do the opposite.
- Consider melatonin cautiously. A 2002 Cochrane Review found melatonin can help combat jet lag, though it called for more research into its safety. The U.S. Centers for Disease Control and Prevention includes melatonin guidance in its travel recommendations, while Britain’s National Health Service does not endorse its use, citing insufficient evidence. Melatonin has been linked to short-term dizziness and nausea, as well as a higher long-term risk of heart failure and death from other causes. Experts recommend consulting a doctor before use, and note that incorrect timing can worsen jet lag.
- Manage light exposure. Controlling exposure to light after arrival can help reset the body clock, but timing matters, travelers may need sunglasses at certain times to avoid unwanted light exposure.
- Exercise at low intensity. Small studies suggest light to moderate exercise combined with light exposure can help shift the body clock, a strategy commonly recommended for professional athletes.
- Eat on local time. Adjusting meal times to match the destination, even during the flight, can help realign the body’s metabolic clock, researchers say.
After long trips, travelers are increasingly opting not to be pampered, but instead to have the jet lag beat out of them with intense, even painful spa treatments that aim to shock your body into equilibrium. https://t.co/Ou7QLvk8ml
— The Wall Street Journal (@WSJ) July 22, 2026
Mindset Matters, Too
An unpublished, not-yet-peer-reviewed study from the University of Surrey found that infrequent long-haul travelers who were more anxious about developing jet lag reported worse symptoms, a possible nocebo effect, in which negative expectations worsen outcomes.
Researchers note the body does adjust naturally over time, typically at a rate of about one hour per day after eastbound travel and about an hour and a half per day after westbound travel.
