s 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 dismiss the [observational] studies and workplace studies that show night work is a health risk,” he says. “We’ve shown unhealthy mechanisms can occur even in healthy subjects, so the time for dismissing night work as a health risk is well past.”
The same is true for jet lag, especially if it’s persistent, such as for pilots and other flight personnel. Buxton is hoping to study pilots in training before they start regularly crossing time zones, to see if the same effects he found in the lab also appear in the real world. In the meantime, it’s worth noting that while much of the attention for preventive health has focused on diet and exercise, sleep is an important part of that equation. “If you’re not getting enough sleep, then it’s hard to find the energy to exercise,” says Buxton. “And if you’re not getting enough sleep, you not only eat more than you need but you tend to make poorer food choices. So in terms of obtaining optimum health, all three pillars — diet, exercise and sleep — are important.”