Anticipation is sweet. In anticipation of the blooming light, plants unfurl their leaves. For many marine creatures, rising to the sea surface as the moon rises is the anticipatory signal that food is available. In our gut, too, microbes anticipate dinnertime because microorganisms have internal clocks that sound the dinner bell.

“And here’s where it gets interesting,” says Hans Van Dongen, a professor of psychology at Washington State University Spokane and internationally known sleep expert.

“The biological clock those organisms have and the brain-based clock that humans have are not necessarily in sync. You notice this when you travel to another time zone. The first trouble people get when they travel to another time zone, other than being sleepy, is that they get GI upset.”

But isn’t that due to the water? That’s the usual suspect. Van Dongen shakes his head.

“Most places have upgraded the quality of their water, and yet we still have these same problems,” he says. “The symbiotic relationship between our microbiomes and ourselves is everywhere but it is particularly strong in the GI system. So just because you travel to another time zone doesn’t mean that the microbiome got the message. So now their clocks and their activities, the proteins that they produce, are out of sync with you.”

Just as our biological clock is governed by certain genes—with telltale names like clock, time, and per (for “period”)—so too bacteria and other microorganisms have homologous genes. But instead of being light-triggered, as is the clock in the suprachiasmatic nucleus of the mammalian brain, microbial communities are triggered by the availability of nutrients in the gastrointestinal tract.

Van Dongen cautions that attributing the negative effects of travel to the microbial clock is not proven. But there is strong evidence that out-of-sync clocks are giving us fits.

“Shift workers,” Van Dongen says, “consistently complain of their GI systems getting out of whack.”

In one longitudinal study of nearly 75,000 nurses, those who worked night shifts on a rotating basis were 11 percent more like to die early compared to those who never worked the night shift. While disruption of the sleep cycle typically takes the blame, another study with mice that were artificially jetlagged confirmed the disruption to their microbiomes. In a followup with humans, researchers confirmed that, indeed, the composition of the gut biome changed after a time zone-jumping trip. Immediately after the trip, the subjects had a higher proportion of Firmicutes, a group of bacteria that, in overabundance, is linked to obesity.

Again, Van Dongen cautions that “we don’t know this as a fact. It’s just an emerging idea.”