With commentary by lead study author Elizabeth Cespedes, ScD, ScM, of Harvard’s T.H. Chan School of Public Health and Kaiser Permanente's Division of Research.
Women who increase their sleep by two hours or more a night over time face a 15% higher risk of developing type 2 diabetes than women who regularly sleep between 7 and 8 hours a night, concludes a recent study in Diabetologia, the journal of the European Association for the study of Diabetes.
The report, which is the first and largest of its kind and involved the landmark Nurses’ Health Study, also corroborates growing evidence from around the world that too little sleep—in this case, 6 hours or less a night—significantly raises the risk of the disease.
“Women who reported chronic short sleep or large increases in sleep had a higher risk of diabetes,” said the study’s lead author Elizabeth Cespedes, ScD, ScM, of Harvard’s T.H. Chan School of Public Health and Kaiser Permanente's Division of Research.
In their study of 59,031 women 55-83 years of age, the researchers compared changes in sleep duration at two intervals over a 14-year period—once in 1986 and again in 2000. While all the women were diabetes-free in 2000, by 2012, 3,513 had been diagnosed with the illness. Two groups faced the greatest risk: Those who reported short sleep at both time points and those who first reported short sleep but later reported longer sleep.
The latest study reached conclusions similar to those documented by the Whitehall II study, which looked at sleep duration reported by 10,308 British civil servants aged 35-55 years over the course of 4, 5-year cycles spanning more than 20 years. Diabetes risk was greatest in those who averaged 5.5 or fewer hours of sleep a night as well as in those whose sleep increased by more than two hours a night.
“In addition to replicating the findings of our own smaller study, the study in Diabetologia adds to our knowledge by taking into account a large number of factors that could cause the sleep-diabetes association to be spurious,” said Whitehall’s lead author Jane Ferrie, Ph.D., of the School of Social and Community Medicine at the University of Bristol, UK.
Indeed, the link between sleep and diabetes held up in the Nurses’ Health study even after researchers adjusted for factors like weight gain, decrease in physical activity and quality of diet, and shift work, given that diabetes risk has been shown to increase most after five years of shift work.
Still, explanations for the longer sleep-diabetes connection remain unclear. Possibilities include everything from detrimental effects of too much time in bed to mood disorders, sleep apnea and chronic illness. Possibly even more important: Longer sleep could be the effect rather than the cause of illness. Longer sleep could signal the pre-symptomatic stages of diabetes itself, Ferrie said.
The harm of too little sleep is better understood: Chronic short sleep has been associated with glucose intolerance, insulin resistance and impaired insulin secretion, all of which increase the risk of type 2 diabetes. The benefit of adequate sleep appears critical not only in preventing diabetes but also in treating it. Recent research has shown that people with diabetes who sleep 6-8 hours a night have much better glycemic control than those who sleep less than six.
“Long-term maintenance of healthy sleep duration is a pillar of health and chronic disease prevention,” Cespedes said. A recent consensus statement by the American Academy of Sleep Medicine and Sleep Research Society recommends that adults regularly sleep seven or more hours a night for optimal health, noting that sleeping less is associated with a number of health problems including weight gain and obesity, diabetes, hypertension, heart disease, stroke and depression.