I’m not a “studies show this, studies show that” kind of guy, but I think this one is important: After just two weeks of a shortened sleep schedule, insulin resistance and impaired glucose tolerance developed in healthy volunteers.
Published in the Journal of Clinical Endocrinology & Metabolism, the study, out of the University of Chicago, reports on 11 volunteers. Their mean age was 39 and their average body-mass Index (BMI) was 26.5 (a BMI in the “overweight” range). The experimental group was allowed to spend no more than 5.5 hours per day in bed. The other group was allowed to remain in bed about 8.5 hours a night.
The test lasted 14 days. Volunteers in both groups were discouraged from exercising and encouraged to eat as much as they wanted. This was to mimic Western lifestyles, according to lead researcher Plamen Penev, MD, PhD.
After at least three month’s time off, the groups were reversed. The low-sleep group was allowed to get more sleep and followed the same eat-anything diet and low-exercise plan, while the other group was told to sleep less. (This format is called a “crossover design,” because both groups get to try both arms of the trial.)
Body-mass index and weight both increased more during the shortened-sleep phase — BMI by 0.7 points, weight by 2.3 kilograms (roughly 5 pounds). The 2-hour glucose tolerance test results for people in the short-sleep groups averaged 144 mg/dl, compared with 132 mg/dl in the longer-sleep groups. Other tests showed reduced insulin sensitivity in the low-sleep groups.
“When part of a Western-like lifestyle, recurrent sleep restriction can also result in reduced insulin sensitivity,” researchers said.
It’s not the first time science has linked poor sleep to diabetes. An article in the December 2007 edition of the journal Sleep reported that sleeping an average of five hours or less or nine hours or more per night increased the risk for Type 2 diabetes. This was based on data from almost 9,000 people self-reporting on their sleep for over ten years. The long or short sleepers had an almost 50% greater chance of developing diabetes than those who averaged seven hours of sleep. This study used data from the first NHANES, or National Health and Nutrition Examination Survey, one of the biggest studies in the world on the effects of lifestyle.
What Does This Mean?
Scientists admitted that they had no idea how lack of sleep contributed to diabetes. They thought that too much sleep might indicate that inflammation was already occurring, perhaps leading to diabetes. But they couldn’t explain the low-sleep connection.
I’m highly suspicious that the connection has to do with stress. In the University of Chicago study, shortened sleep was accompanied by a 25% increase in nighttime norepinephrine levels. Norepinehprine is a stress hormone. And we know that stress contributes to insulin resistance and diabetes.
I think we should all try to get our 7–9 hours of sleep each day. Unfortunately, a July 2006 report in the American Journal of Epidemiology reports that the average American gets less than 7 hours of sleep nightly. That means that many of us get even less. The study found that people with lower income slept less. Men slept less than women, and black people slept less than white people. Black men averaged only 5.1 hours of sleep, which is classed as “sleep deprivation.” Interestingly, these are the same patterns you see in overall health and longevity.
So unless you’re rich (and white), you’re probably not getting enough sleep. And maybe not even then. Although the people in this study spent an average of 7.5 hours a night in bed, they spent only 6.1 hours asleep.
We don’t know if it’s the lack of sleep itself that causes problems. It might be that the stress that keeps you awake also causes insulin resistance. But studies suggest that average sleep times have declined since 1900, when people reported sleeping nine hours a night. Studies from the 1970s found self-reported average sleep times closer to seven hours. And the newest study shows that actual sleep (measured with a wrist meter) was closer to six hours.
So what to do? Obviously, we want to reduce stress and get more sleep. Unless you’re really swamped with work and family duties, you should have time for seven hours’ sleep. It might take some work, but let’s face it. As health recommendations go, “sleep more” is a pretty pleasant one.
Remember, there’s no danger of overdoing it. Unless you’re seriously depressed or have another underlying medical condition such as a sleep disorder, you can’t really oversleep. Your body just won’t sleep when it’s not tired. The place to start is by keeping a sleep log to find out how much you’re actually sleeping now. Then see how you can increase that time. Let us know how it goes. I’ll be right there with you — I think I really need to practice what I’m preaching on this one.