People who are constantly jet-lagged or work night shifts are especially prone to colds, infections, and other inflammatory diseases. Now experts know exactly why this is the case. According to a new study, our circadian clocks – the body's natural response to light cues, which tells us when to wake up or wind down – plays a huge role in immunity.
"We discovered that the same clock mechanism that senses time of day in order to rev up or shut down bodily processes, such as digestion, also regulates the development of key cells in the immune system," says study author Lora Hooper, a professor of immunology at University of Texas Southwestern Medical Center in Dallas. "We think this could explain why people who fly overseas a lot or who work a string of overnight shifts and then try to readjust to normal hours tend to get more inflammatory diseases."
Hooper and her team uncovered this connection by studying what would happen if they gave rats "jet lag" by manipulating light cycles in the lab. They found that the overtired rats had a high number of immune cells. That's a problem: "These immune cells, which dwell mostly in the gut, are basically soldiers on the front lines, protecting the body against invasive bacteria and viruses," she says. "When there are too many of them, however, they start running amok, damaging tissues, and causing inflammation, which then makes you sick."
According to Hooper, the jet-lagged rats became overrun with these cells because their circadian clocks, which would've normally told their bodies to stop making new cells, had been thrown so out of whack. She can't guarantee that this is also what goes on in overtired humans' bodies to make them sick, but she's pretty confident it's a factor.
"If you're overnight work schedule is one week on, one week off, and you're continuously veering from one to the other, then your clock is really screwed up," she says. "Same thing if you fly to Europe often and are always trying to acclimate to the time difference. Disrupting the circadian clock seems to really affect the regulation of these immune cells, which appears to contribute to illness.