During the pandemic, many people barely left their neighborhoods, let alone their own time zones. But vaccines are available, cabin fever is rampant, and the holiday travel season is upon us. And so, inevitably, is jet lag.
The human internal time-keeping apparatus, known scientifically as the circadian clock, is a powerful force. It synchronizes functions across organs and tissues, and affects cognitive function, digestion, sleep, and even asthma. Adjusting the circadian clock to a new time zone or schedule isn’t as simple as resetting a wristwatch, but current research about how to manipulate it can be helpful for anyone, whether they’re traveling to their in-laws’ house or to Mars.
“There’s so much promise coming forward, now that we understand the molecular power of the clock, to harness the power of the clock for good,” says Carrie Partch, a professor of biochemistry at UC Santa Cruz who studies the circadian system. She says the more we understand about the clock, the more freedom we’ll have, because we can make it an ally rather than a foe.
Throughout the body, cells have their own circadian clocks that regulate metabolism and other cellular functions. Those clocks coordinate between other cells in specific organs and even between organs—though how they do that is something scientists are still trying to figure out. All of these individual clocks are regulated and synchronized by the brain’s suprachiasmatic nucleus, a “pacemaker” part of the hypothalamus that is highly sensitive to external stimuli, specifically light and darkness. Light signals it’s time to wake up and be alert, while dark means that it’s time to slow down and sleep.
While those signals are intensely tied to the sleep cycle, they have downstream effects on a host of biological functions. “I think of the circadian pacemaker as the conductor of an orchestra,” says Erin Flynn-Evans, who leads the NASA Ames Research Center Fatigue Countermeasures Laboratory. “It controls a whole concert of biological function. There are circadian clocks in the liver, in the gut, in reproductive hormones. The master pacemaker in the suprachiasmatic nucleus is sort of synchronizing the timing of all that biological function.”
But that internal timekeeper can’t always keep up with human behavior. When travelers move quickly across time zones, the circadian clock gets desynchronized from the exterior world, an experience most people know as jet lag. That mismatch can cause a range of symptoms including fatigue and bleariness, insomnia, and even digestive problems.
For most people, it’s a relatively rare event and just an inconvenience. But for workers like pilots and flight attendants, who may endure these changes daily, jet lag can affect their long-term health. Even relatively short hops affect cognitive function. One 2017 study published by researchers at Northwestern University found that professional baseball players who traveled over just two or three time zones for a game played worse. The same problems exist for shift workers like nurses, and people with irregular hours like long-haul truck drivers, who operate on schedules that keep them awake at night.