(dailyRx News) Too little sleep or too much poor quality sleep can hurt more than your energy levels - it can also hit your waistline. If you're not getting enough sleep, you may be trading Z's for pounds.
A recent article that reviews the evidence from various sleep studies discusses patterns found across the studies linking sleep deprivation and obesity risk factors.
Kristen Knutson, PhD, an assistant professor of medicine at the University of Chicago, gathered evidence from both observational and experimental sleep studies.
In observational studies, researchers do not manipulate the conditions of the participants but simply observe activities and associated patterns. Knutson focused on those that looked at links between getting fewer than six hours of sleep and obesity, or having a higher body mass index score.
Most of the studies Knutson reviewed were done in Western countries such as the U.S.
She found that diet and physical activity may not be the only factors in contributing to obesity. A number of the studies noted sleep's impact on two hormones related to appetite stimulation and suppression called leptin and ghrelin.
Leptin is released by fatty tissue and signals to the body that the person has had enough to eat. Meanwhile the hormone ghrelin, released from the gut, stimulates the appetite.
If the release of these hormones is disrupted, as reduced amounts of sleep appears to do, a person may end up eating extra food without burning off enough of the extra calories.
Insulin is another signaler, released by the pancreas after a meal, that reduces a person's appetite and that has been found to be affected by too little sleep.
"A review of the evidence shows how short or poor quality sleep is linked to increased risk of obesity by de-regulating appetite, leading to increased energy consumption," Knutson said.
For example, in one 2004 study Knutson looked at, healthy men who were restricted to four hours of sleep had leptin levels that were 19 percent lower than when they were given 12 hours a night in bed. Yet their physical activity and calorie intake were unchanged during both times.
Other experimental studies showed similar results where leptin levels were lower - sometimes by up to 33 percent - in people who had been deprived of sufficient sleep.
Knutson said the 53 million American adults that the National Sleep Foundation estimates get fewer than six hours of sleep - almost one out of every five adults - may be at an increased risk for obesity.
Knutson cites past studies that found the link between insufficient sleep and a higher body mass index to be more pronounced in children and adolescents and to be stronger among lower socioeconomic groups.
"Several studies have reported that inadequate sleep is more common among those with lower socioeconomic status," she wrote.
The observational studies Knutson looked at also revealed links between insufficient sleep and a greater risk for cardiovascular disease and diabetes, especially among those suffering from sleep apnea.
However, Knutson writes that it was not possible to determine whether one condition, such as insufficient sleep or sleep apnea, caused another, such as diabetes, or vice versa or neither.
Other limitations that existed in the studies were that many of the studies were observational and used self-reporting instead of actigraphs, a device to determine exactly how much sleep a person actually got.
Also, not all the studies took into account other factors that might influence cardiovascular risk, such as race, consumption of alcohol or caffeine, amount of physical activity, socioeconomic status and underlying psychiatric conditions.
Knutson suggests additional studies in other parts of the world that use objective measures of sleep to provide better evidence regarding links to cardiovascular and metabolic disorders, such as diabetes.
Overall, however, Knutson did a very good job of surveying the evidence and assessing what is known about links between poor sleep and obesity risks, according to William Kohler, MD, the director of the Florida Sleep Institute in Spring Hill.
"This was an excellent review with potential mechanisms pointed out as to why there is an association between sleep deprivation and obesity and metabolic disorders," Dr. Kohler said.
A chart in Knutson's paper illustrated various ways in which inadequate sleep might contribute to obesity, such as more time available for eating, higher levels of ghrelin and lower levels of leptin, a lower use of brain glucose, higher levels of the stress hormone cortisol, and reduced levels of energy expenditure.
Having more time to eat and imbalances in leptin and ghrelin, for example, could lead to increased food intake while imbalances in brain glucose and nighttime cortisol levels could alter a person's glucose metabolism.
Together, these possible effects, along with others detailed in the paper, could lead to obesity and/or insulin resistance, which can develop into type 2 diabetes.
The higher risk of cardiovascular disease could result from higher blood pressure levels that can occur with the increased activity in the sympathetic nervous system observed in those who don't get enough sleep.
The experimental studies Knutson looked at were limited mostly in terms of how long they ran because of the ethics of depriving people of sleep for too long.
"Although several studies have examined the impact of a night of total sleep deprivation on physiology, the effects are usually temporary because total sleep deprivation cannot be maintained for very long," she wrote.
Although the link between sleep deprivation and obesity was evident in the body of studies, there was not enough research available to determine whether improving a person's sleep could help prevent obesity.
The most important take-away from the review, according to Dr. Kohler, is that the public health and medical communities need to continue to educate the public about the importance of sleep.
"For so many years, we've taken sleep for granted and gotten what we could when we could," he said. "But we need to pay more attention to sleep and realize the potential consequences to our lives both physically and emotionally if we do not get adequate sleep."
The research appeared in a special issue on obesity published in the American Journal of Human Biology that includes articles from January through March.
This study was funded by a grant from the National Heart, Lung and Blood Institute of the National Institutes of Health. Knutson stated she had no conflicts of interest.