Harvard Women's Health Watch

In the journals: Scientists discover how shift work may threaten health

People who work on night or rotating shifts are at increased risk for obesity, diabetes, and cardiovascular disease. The biological mechanism leading to such work-related health outcomes has been largely unknown — until now. A study has found that a mismatch between the body's circadian rhythms and behavioral "rhythms" such as the sleep/wake and eating/fasting cycles causes abnormal changes in hormonal and metabolic functions. Such a mismatch — called circadian misalignment — may also explain jet lag. Results were published in the March 17, 2009, issue of the Proceedings of the National Academy of Sciences.

To investigate the physiological effects of circadian misalignment, researchers at Boston's Brigham and Women's Hospital studied 10 healthy volunteers for 10 days in a laboratory free of time cues. The subjects were placed on a 28-hour day, each day beginning four hours later than the one before, until their sleep/wake cycles were reversed. Activities and meals were also carefully controlled. As a result, the volunteers slept and ate at all phases of their internal circadian cycles, or body clocks. The researchers took hourly measurements of glucose, insulin, cortisol (the so-called stress hormone), and leptin (an appetite-regulating hormone that sends fullness signals to the brain) and daily measurements of blood pressure, heart rate, and other functions, including brain wave activity during sleep.

Among the findings: circadian misalignment lowered leptin levels (a factor contributing to increased appetite and possibly obesity), raised waking blood pressure, and increased glucose (despite increases in insulin levels). Three of the 10 subjects showed signs of prediabetes. Circadian misalignment also disrupted normal cortisol cycling, which the authors say could contribute to insulin resistance.

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