Sleeping during the day and staying awake at night can lead to heart and metabolic problems, even after just a few days of the out-of-sync schedule, a new study reports. The results, published online March 2 in Proceedings of the National Academy of Sciences, may help explain the high rates of heart disease, diabetes and obesity among people who work the graveyard shift.

“The problems of shift work affect so many people, but there are very few studies that address the underlying mechanisms,” comments Eve Van Cauter, a sleep researcher at the University of Chicago. “This is what [the researchers] have done, and elegantly so.”

Based on Bureau of Labor Statistics data, the researchers estimate that 8.6 million people in the United States are shift workers.

To figure out what happens when people are moved from their normal sleep/wake cycle, Frank Scheer of Brigham and Women’s Hospital in Boston and his colleagues imposed a 28-hour day on study participants. “The biological clock ticks along at its own rate,” Scheer says, and that rate, not so coincidentally, is right around 24 hours for most people.

Some people can adjust to day periods that are slightly longer or shorter than 24 hours and, with the right light cues, can also adjust to a shifted 24-hour sleep/wake cycle. But studies have shown that people cannot acclimate to a 28-hour cycle.

For this study, five men and five women lived out 28-hour cycles in dimly lit, private rooms with no windows to clue the volunteers in to the real time. Over the course of the 10-day experiment, the total amount of shut-eye did not vary: Participants slept nine hours and 20 minutes each 28-hour day (instead of eight hours every 24-hour day). But the times of sleep varied. Each “day,” volunteers went to bed four hours later, in real-world time, than they had the previous “day.”

When the volunteers were awake and active in the middle of the night — when their bedtime had shifted by 12 hours — researchers noted a significant spike in blood pressure, a decrease in the appetite-regulating hormone leptin and higher–than-normal blood sugar levels. Three of the 10 subjects, who were all previously healthy, had blood sugar that reached prediabetic levels.

“Even after just a few days, they showed striking changes in glucose metabolism,” Scheer says. "The rapid onset within just a few days shows that this may even temporarily affect the millions of people experiencing jet lag every year."

Because the total amount of sleep time did not change, researchers concluded that these harmful effects stem from an off-kilter sleep cycle, and not from simply too little sleep.

“This experiment demonstrates that misalignment in and of itself is critical,” comments Charles Czeisler, a sleep expert at Harvard Medical School.

The new research didn’t measure the quality of sleep, which may be responsible for some of the harmful effects of shift work, Van Cauter says. “Shallow, fragmented sleep could be the mediator.”

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