Increasing sleep duration over time may raise the risk for type 2 diabetes in middle-aged and older women, and doing this as an antidote to shorter sleep duration earlier in life may not ameliorate the adverse effects of the latter, a new study suggests.
The findings, from the longitudinal Nurses’ Health Study, were published online November 2 in Diabetologia by Elizabeth Cespedes, ScD, MSc, research postdoctoral fellow, at Kaiser Permanente division of research, Oakland, California, and colleagues.
The analysis of 59,031 women is the largest ever to assess the relationship between long-term changes in self-reported sleep duration and changes in energy balance and subsequent type 2 diabetes risk.
Both increases of 2 or more hours a day of sleep between the years 1986 and 2000 and consistent sleep duration of 6 hours a day or less were associated with an increased risk for subsequent type 2 diabetes, but the short sleep risk lessened after adjustment for confounders.
“Our results support the message that long-term maintenance of healthy sleep duration is a pillar of health and chronic disease prevention,” Dr Cespedes said.
“However, simply increasing sleep duration after previous years of short sleep may not be a panacea,” she stressed. “This study reinforces previous research about the importance of sleep in helping to maintain a variety of indices of good health, including obesity and diabetes,” she added.
U-Shaped association between sleep and diabetes: constant sleep duration best
In the study, the overall average sleep duration remained constant between 1986 and 2000. Half of women (49%) reported getting about 7 to 8 hours of sleep at both time points, while 15% consistently slept 6 or fewer hours a night, and about the same proportion slept 9 or more hours.
Only 5% reported decreases of 2 or more hours’ sleep duration and just 7% reported increases of the same amount between 1986 and 2000.
Compared with women whose sleep duration didn’t change, those with either increases or decreases of 2 or more hours had higher body mass indexes (BMIs), less physical activity, and poorer diet quality. They were also more likely to snore, to smoke, to take antidepressants, and to have hypertension, hypercholesterolemia, and a family history of diabetes. More of them had also performed shift work for 5 or more years.
Women who reported sleep duration increases of 2 or more hours a day were also slightly older and gained more weight from 1986 to 2000.
A total of 3513 women developed type 2 diabetes from the study start through 2012. There was a U-shaped relationship between changes in sleep duration and diabetes, with both decreases and increases associated with greater diabetes risk.
However, after adjustment for sleep duration in 1986, race/ethnicity, diabetes family history, shift work, hypercholesterolemia, hypertension, menopause, snoring, sleep apnea, antidepressant use, smoking, alcohol use, diet, physical activity, and BMI, only increases in sleep duration remained significantly associated with diabetes risk (hazard ratio, 1.15).
“When we took into account physical activity, diet quality, and body mass index, which are mechanisms through which sleep might influence diabetes risk, all of these relationships weakened,” said Dr Cespedes.
“Yet the relationship of large increases in sleep duration to subsequent risk of diabetes remained significant,” she explained.
The finding that chronic short sleep duration increases the risk of diabetes but that this excess risk is explained by BMI is consistent with much of the literature, she noted.
On the other hand, the biologic rationale for why long or extended sleep duration might influence diabetes is not as clear. Some scientists argue that long sleep is a symptom of an underlying sleep disorder, depression, or ill health and that it is these factors and not long sleep per se that increase risk of diabetes.
“However, in our study, we controlled for many of these indicators of health status, including sleep apnea diagnosis, snoring, and depression, and we still saw a relationship between large increases in sleep duration and increased risk of diabetes. One possibility is that when people report longer sleep they are spending more time in bed but not necessarily sleeping more.”
Catching up later in life doesn’t work
Dr Cespedes and colleagues note that their findings with regard to the detrimental impact of chronic short sleep duration on metabolic health are consistent with previous literature and “newly suggest that the adverse influence of short sleep may not be ameliorated through increases in sleep duration later in life.”
“However, we did not have information on why changes in sleep duration occurred,” they point out.
Two ongoing intervention trials may help further inform this topic, they note.
One, a study among chronically short-sleeping, obese adults, could provide insight about the benefits of intentionally extending sleep duration (Clin Trials. 2010;7:274-285.)
Another study will test whether moderate sleep restriction in long-sleeping older adults improves glucose tolerance and inflammation (Contemp Clin Trials. 2013;36:175-186.)
Source: Diabetologia. Published online November 2, 2015. Abstract