Our species’ waking and sleeping cycles–shaped in millions of years of evolution–have been turned upside down within a single century with the advent of electric lighting and airplanes. As a result, millions of people regularly disrupt their biological clocks–for example, shift workers and frequent flyers–and these have been known to be at high risk for such common metabolic diseases as obesity, diabetes, and heart disease. A new study published in Cell, led by Weizmann Institute scientists, reveals that our biological clocks work in tandem with the populations of bacteria residing in our intestines, and that these microorganisms vary their activities over the course of the day. The findings show that mice and humans with disrupted daily wake-sleep patterns exhibit changes in the composition and function of their gut bacteria, thereby increasing their risk for obesity and glucose intolerance.

A consensus has been growing in recent years that the populations of microbes living in and on our bodies function as an extra “organ” that has wide-ranging impacts on our health. Christoph Thaiss, a research student in the lab of Dr Eran Elinav of the Weizmann Institute’s Immunology Department, led this research into the daily cycles of gut bacteria. Working together with David Zeevi in the lab of Prof Eran Segal of the Computer Science and Applied Mathematics Department, and Maayan Levy of Elinav’s lab, he found a regular day-night cycle in both the composition and the function of certain populations of gut bacteria in mice. Despite living in the total darkness of the digestive system, the gut microbes were able to time their activity to the mouse’s feeding cycles, coordinating daily microbial activities to those of their host.

Does this finding have any medical significance? To further investigate, the researchers looked at “jet-lagged” mice, whose day-night rhythms were altered by exposing them to light and dark at different intervals. The jet-lagged mice stopped eating at regular times, and this interrupted the cyclic rhythms of their internal bacteria, leading to weight gain and high blood sugar levels. To verify these results, the scientists transferred bacteria from the jet-lagged mice into sterile mice; those receiving the “jet-lagged microbes” also gained weight and developed high blood sugar levels.

The research group then turned to human gut bacteria, identifying a similar daily shift in their microbial populations and function. To conduct a jet-lag experiment in humans, the researchers collected bacterial samples from two people flying from the United States to Israel—once before the flight, once a day after landing when jet lag was at its peak, and once 2 weeks later when the jet lag had worn off. The researchers then implanted these bacteria into sterile mice. Mice receiving the jet-lagged humans’ bacteria exhibited significant weight gain and high blood sugar levels, while mice getting bacteria from either before or after the jet lag had worn off did not. These results suggest that the long-term disruption of the biological clock leads to a disturbance in their bacteria’s function that may, in turn, increase the risk for such common conditions as obesity and imbalances in blood sugar levels.

Segal says in a release: “Our gut bacteria’s ability to coordinate their functions with our biological clock demonstrates, once again, the ties that bind us to our bacterial population and the fact that disturbances in these ties can have consequences for our health.”

Elinav says: “Our inner microbial rhythm represents a new therapeutic target that may be exploited in future studies to normalize the microbiota in people whose life style involves frequent alterations in sleep patterns, hopefully to reduce or even prevent their risk of developing obesity and its complications.”