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How night shifts disrupt metabolism

Being awake at night and asleep during the day can disrupt the rhythms of certain metabolic pathways. Pranodhm/iStock/Thinkstock

You wake up, feel hungry, and fall asleep each day around repeating 24-hour “circadian” cycles controlled by your body’s internal clocks. These clocks are synchronized by a central pacemaker in the brain. Cycles of light and dark are important for the function of the brain’s master clock. Other cycles, such as the behavioral activities of eating and fasting or sleeping and waking, are important for peripheral clocks in the liver, gut, and other tissues.

When you stay awake all night or otherwise go against natural light cycles, your health may suffer. Long-term disruption of circadian rhythms has been linked to obesity, diabetes, and other health problems related to the body’s metabolism.

Previous studies have shown that some metabolites—the products of metabolism—in blood can have daily rhythms. An international research team led by Drs. Hans P. A. Van Dongen and Shobhan Gaddameedhi at Washington State University investigated whether disruptions in these rhythms are influenced by the central pacemaker in the brain or reflect behavioral activities, such as working the night shift. The study was funded in part by NIH’s National Institute of Environmental Health Sciences (NIEHS). Results were published online in the Proceedings of the National Academy of Sciences on July 10, 2018.

Ten men and four women, aged 22 to 34 years, stayed at a research lab for one week. Half had a night-shift sleep pattern for three days and half had a day-shift pattern. The night-shift pattern causes the central pacemaker and behavioral rhythms to be at odds. After three days, the volunteers were kept awake for one day in a constant routine with a constant level of temperature and light. They received identical snacks every hour and provided blood samples every three hours.

The research team found only small differences in the day-shift and night-shift patterns for melatonin and cortisol, which mark the activity of the brain’s master clock. This finding suggests that the master clock is resistant to influence from the night-shift pattern.

The team analyzed the levels of 132 metabolites during the 24-hour constant routine. About half (65) of the metabolites had a significant daily rhythm. Of these, 27 had a significant 24-hour rhythm for both sleep patterns. Only three of these metabolites (taurine, serotonin, and sarcosine) kept the same peak time, similar to the master clock markers melatonin and cortisol. The other 24 showed a 12-hour shift in rhythm for the night-shift pattern.

The researchers noted that the particular metabolites and pathways affected by the night-shift sleep pattern relate to the liver, pancreas, and digestive tract. These findings suggest that night-shift sleep patterns can disrupt certain metabolite rhythms and the peripheral clocks of the digestive system without affecting the brain’s master clock.

“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,” Van Dongen says. “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.”

Further research is needed to better understand the role of these metabolic pathways in obesity, diabetes, and other medical conditions for which shift workers are at increased risk.

—by Geri Piazza