Studies show that shift work and other sleep disturbances like jet lag can disrupt your body clock and increase the risks of obesity and diabetes. But, until now, researchers haven’t really been sure exactly how these changes affect the body’s metabolism.
To find out, Dr. Orfeu Buxton, an assistant professor in the division of sleep medicine at Harvard Medical School, and his colleagues invited 21 men and women to participate in a study in a controlled laboratory setting, where they would have their sleep-wake cycles purposefully disrupted. Over the course of five weeks, the researchers determined when and how much the participants slept, ate and exercised. Although lab-based studies have previously examined the health effects of interferences with the body’s natural circadian rhythm, most of those trials have lasted only about a week or two.
Buxton, who is also an associate neuroscientist at Brigham and Women’s Hospital, was interested in analyzing the effects on metabolism and diabetes risk of longer-term, or chronic, changes to the circadian clock — such as those experienced by shift workers and frequent travelers.
After an initial period of adequate sleep, in which the men and women spent 10 hours in bed each night, Buxton and his team gradually started to dial back the amount of the participants’ slumber. In addition to getting less sleep, the study volunteers were also asked to go to bed and wake up four hours later each day while in the lab, a technique devised to mimic traveling one east-west time zone per day. Eventually, the participants found themselves sleeping during the day and waking at night, just as shift workers do. “We imposed a disrupted cycle and schedule of activities, from sleeping, waking, eating and fasting that was in misalignment with their endogenous 24-hour circadian clock,” says Buxton. “Nobody would do a night shift as punishing as this.”
What Buxton and his colleagues found, however, was worth the trouble. The resting metabolic rate of the volunteers by the end of the five weeks was 8% lower than where they had started. Over a year, says Buxton, such a slowdown of metabolism could translate into a 10-lb. weight gain if people’s diet and exercise habits remained the same. That could explain why night shift workers tend to gain more weight and have a higher likelihood of obesity than day workers; such weight gain is linked to an increased risk of diabetes and heart disease.
The scientists learned something else interesting about another mechanism that put the disrupted sleepers at higher risk of diabetes: the combination of having their circadian clocks reversed (sleeping during the day and waking at night) and the poorer sleep they got as a result had an effect on their insulin levels. After three weeks in the lab, the participants produced about a third less insulin from the pancreas in response to meals; with less insulin available to break down glucose, blood glucose levels started to rise and three of the 21 volunteers showed high enough levels to qualify them as pre-diabetic. (Fortunately, during the nine day washout period at the end of the study, during which participants were allowed to resume their natural day-night cycles, their glucose levels went back down to normal.)
The findings suggest that circadian disruptions combined with inadequate sleep affect the body differently than does sleep deprivation alone. In previous studies in which volunteers slept less, but maintained their natural sleep-wake cycles, they showed less sensitivity to insulin, but there was no change in the amount of insulin released by the pancreas. In the current study, adding changes to the circadian clock seemed to have a more profound effect on the pancreas. Both pathways can contribute to an increased risk of diabetes, but in different ways, says Buxton. “The body responds to sleep restriction in one way, and circadian disruption combined with sleep restriction in a very different way,” he says.
The findings highlight how important the sleep-wake cycle can be for metabolism and health. “It’s harder to di