The study, by American and Japanese researchers, paints the clearest picture yet of how the body’s internal clocks schedule a complex range of functions over a 24-hour period.
In order for the body to stay healthy, different systems must peak and ebb at precise times. The endocrine glands ratchet up production of key hormones in the morning. The immune system pumps out microbe-hunting lymphocytes at night.
Scientists thought all these processes were kept in line by a master clock in the brain, which adjusts itself to seasonal or travel-induced changes in daylight.
The researchers knew a crucial gene in rats, called Per1, helped the master clock regulate these processes. But given the numerous systems the clock must control, the team speculated that Per1 might run smaller clocks in other parts of the body, like the liver and lungs.
In order to test this, the researchers shifted the schedules of rats so that they received an extra six hours of day or night — the equivalent to flying between London and New York.
After this, the researchers removed and kept alive tissues from key parts of the of rats’ bodies, including their master clocks, and recorded the circadian rhythms.
Tissue from the master clock took a day to adapt to the new schedules, said University of Tokyo’s Hajime Tei, an author of the study.
But tissue from the lungs and muscles took at least six days to adapt to the time shifts, and liver tissue took more than 16.
These out-of-sync organs may spell trouble for travelers, according to the University of Virginia’s Michael Menaker, another of the study's authors. "Among other things, the liver plays a role in digestion, and digestive troubles are one of the most common symptoms of jet lag."
Travelers aren’t the only ones at risk. Studies have found that workers on the nightshift or other unorthodox schedules have higher rates of ulcers, heart disease and depression.
Joseph Takahashi, a biologist at Northwestern University, said, "The paper shows that rats take at least six days to get their body clocks back in sync, but most shift workers switch their schedules every week," leaving the body in constant flux. "It might make more sense to change schedules every month," he said.