We have the least risk of getting type 2 diabetes when we get 7 to 8 hours of sleep per night, according to a new study. Many Americans get less than that amount, and it's not hard to accept that we need that much. But what is hard for me to accept is their finding that more than 8 hours of sleep is as bad for us as getting too little sleep.
This study is a meta-analysis of 10 previous studies that Diabetes Care, a professional journal of the American Diabetes Association, just published in its March 2015 issue. Only the abstract of the study, "Sleep Duration and Risk of Type 2 Diabetes," is available free online, but my friend and colleague, Dr. Bill Quick, sent me the full-text.
The study focuses on people who are lucky enough not to have diabetes yet. It doesn't directly tell those of us who already have type 2 diabetes how much sleep is best for us, but understanding how diabetes develops from pre-diabetes makes it clear that the best amount of sleep is the same no matter which condition we have.
How diabetes develops from pre-diabetes
Diabetes comes from the body's inability to metabolize carbohydrates well. For people with type 1 diabetes the problem comes from something that causes the beta cells of the pancreas to stop producing insulin. Type 2 diabetes, which affects 90 to 95 percent of all people with the disease, is more complex. We can describe this type of diabetes as a two-hit disease.
Typically, the first hit is insulin resistance, which people with pre-diabetes have. This makes it more difficult for blood sugar to enter the body's cells. At this point the pancreas still produces quite a bit of insulin, but the body can't use it well enough. In fact, the pancreas pumps out more and more insulin to compensate for using it so poorly.
But after years of trying to make up for the resistance the body has to the insulin that it needs, the beta cells of the pancreas tend to wear out. This beta cell fatigue is the second hit, and only then do you have diabetes.
Sleep's U-shaped curve
The challenging part of this new study doesn't show that more sleep is better. In fact, the study indicates that it's just about as unhealthy to sleep a lot as to sleep a little. Most conditions that medical researchers study show something like a straight line. They call it a "dose-response curve." But sleep seems to follow a "U-shaped curve."
Is this a real problem, or is it perhaps because of something else that makes people stay in bed longer? Studies like this one tell us about association, but not causation, so I find it hard to accept that too much sleep is bad for us.
In fact, "experts caution against studies showing ill effects from too much sleep,"writes Sumathi Reddy in The Wall Street Journal. "I don't think you can overdose on healthy sleep," says Safwan Badr, chief of the division of pulmonary, critical care, and sleep medicine at Wayne State University School of Medicine in Detroit. "When you get enough sleep your body will wake you up."
Some researchers say that because of genetic differences, some of us need more sleep than others. But I think the evidence is clear that almost everyone does better on at least 7 hours of sleep. Whether we have pre-diabetes or diabetes.
See more of my articles on how to manage diabetes:
The Important Role Sleep Plays in Diabetes
Sleep More, Weigh Less for Diabetes Control
The Sleep Apnea-Diabetes Connection