Shift workers with irregular hours are at risk for obesity, diabetes, and cardiovascular disease. A study published online March 2 in Proceedings of the National Academy of Sciences teases apart the influences of body clock and behavior, suggesting that throwing off circadian rhythms negatively affects body chemistry.
Frank Scheer and colleagues at Brigham and Women’s Hospital and Beth Israel Deaconess Medical Center, both in Boston, studied 10 healthy volunteers for 10 days. A “day” consisted of 28 hours, each beginning four hours later than the “day” before, until participants became nocturnal. Biochemical measurements were taken hourly throughout the study period.
Circadian misalignment caused levels of leptin—the hormone that sends signals of “fullness” to the brain—to decrease, possibly raising appetite. Blood glucose and insulin went up, with three previously healthy subjects described as prediabetic. The sleep hormone melatonin was high when people were awake and absent while they were trying to sleep. Patients had more trouble sleeping, and waking blood pressure increased.
By repeatedly skewing the sleep-wake pattern while controlling daily activities, the investigators separated circadian rhythms from “lifestyle” factors such as socioeconomic status, diet, smoking, and sleeping habits. “Our study shows that circadian misalignment causes mechanistic changes that could explain the diseases found in shift workers,” says Scheer.