A new study revealed how working night shifts disrupted metabolism, leading to obesity, diabetes and other metabolic disorders.
The study published on Monday in the Proceedings of the National Academy of Sciences, dispelled the belief that the metabolic disruption in shift workers was driven primarily by the brain's master clock, which normally keeps our bodies on a day-night cycle and uses light cues to synchronise the rhythms of the body's organs and tissues.
Instead, the study found that separate biological clocks (so-called peripheral oscillators) in the liver, gut and pancreas have a mind of their own.
Working with colleagues at the University of Surrey, the Washington State University (WSU) researchers collected blood samples from healthy volunteers who had just completed either a simulated day shift schedule or a simulated night shift schedule.
The investigators analysed the blood samples for metabolites, products of chemical reactions involved in digestion, such as the breakdown and oxidisation of food molecules, as well as in other metabolic processes in cells and organs.
They found that, following the night shift schedule, 24-hour rhythms in metabolites related to the digestive system had shifted by a full 12 hours, even though the master biological clock in participants' brains had only moved by about two hours.
"No one knew that biological clocks in people's digestive organs are so profoundly and quickly changed by shift work schedules, even though the brain's master clock barely adapts to such schedules," said the paper's co-senior author Hans Van Dongen, director of the WSU Sleep and Performance Research Center.
"As a result, some biological signals in shift workers' bodies are saying it's day while other signals are saying it's night, which causes disruption of metabolism," said Dongen.
The research team's work may have implications for the study of other chronic diseases shift workers are more susceptible to, including chronic kidney disease and breast, prostate and skin cancer.
The study included 14 participants who each spent seven days inside the sleep laboratory at the WSU Health Sciences Spokane campus.
First, half of them completed a three-day simulated night shift schedule, while the rest were on a three-day simulated day shift schedule.
Then, after completing their simulated shifts, all participants were kept in a constant routine protocol used to study humans' internally generated biological rhythms independent of any external influences.
During this protocol, they were kept awake for 24 hours in a semi-reclined posture. They received identical snacks every hour and were kept under constant light exposure and room temperature. Every three hours a blood sample was drawn.
The blood samples were analysed at the University of Surrey's Metabolomics Core Facility for 132 different metabolites related to metabolism and the digestive system, said a Xinhua report.
"Twenty-seven metabolites followed a 24-hour rhythm during both the simulated night and day shift schedules," said the paper's first author Debra Skene, professor of neuroendocrinology at the University of Surrey.
"Of these, 24 displayed a dramatic 12-hour shift in rhythm following the simulated night shift schedule, which was not observed following the day shift schedule. This indicated that just three days of being on a night shift schedule has the potential to disrupt metabolism."