Missing out on deep sleep can leave you feeling slow-witted and irritable in the morning, but the consequences don't necessarily end there. Over time, too little deep sleep may also take a toll on your heart by contributing to high blood pressure, a new study suggests.
Healthy young and middle-aged adults spend about 20 percent to 25 percent of their sleeping hours in the stages known as slow-wave sleep (so called because of the brain waves associated with it). This sleep phase is considered restorative and has been shown to be important for memory and mental performance.
The new study, which included 784 men over the age of 65, adds to the growing evidence that slow-wave sleep is also essential to our metabolism and heart health. Compared to men who spent at least 17 percent of their sleep time in the slow-wave phase, those who spent less than 4 percent in this restful state had 83 percent higher odds of developing high blood pressure (hypertension) years later, the study found.
The research should be considered "exploratory," the authors say, and it doesn't prove a direct link between sleep patterns and hypertension. But it suggests that "an important aspect of successful aging is the preservation of good sleep quality," says Eve Van Cauter, PhD, the director of the Sleep, Metabolism, and Health Center at the University of Chicago.
Older people tend to get less slow-wave sleep as they age, and fighting this natural decline -- through healthy sleep habits, for instance -- could be an "extraordinarily important strategy" for heading off hypertension, adds Van Cauter, who wasn't involved in the new research.
Sleep problems have been linked to high blood pressure before. Sleep apnea, a chronic disorder in which a person wakes up struggling for breath several times during the night, is strongly linked to hypertension, although it's not clear whether the disorder causes high blood pressure or vice versa -- or whether the two conditions feed each other.
Untangling the relationship has been tricky in part because obesity increases the risk of both high blood pressure and sleep apnea. Obesity could play a role in the link between slow-wave sleep and hypertension as well; in a previous analysis, the authors of the current research found that insufficient slow-wave sleep was related to obesity.
In the new study, published today in the American Heart Association journal Hypertension, researchers at the University of California–San Diego and Harvard University assessed the sleep quality of the participants using polysomnography, a technique in which electrodes are used to track brain activity.
All of the men had normal blood pressure when they underwent the test, which was performed on a single night in their own beds (as opposed to in a sleep lab). When the researchers followed up with the men an average of 3.5 years later, roughly 31 percent of the study participants had developed hypertension.
Once the researchers took age, body mass index, and race into account, they found that time spent in slow-wave sleep was the only measure of sleep quality associated with hypertension risk. Forty-one percent of the men who got the least slow-wave sleep went on to develop hypertension, compared to 26 percent of the men who got the most slow-wave sleep.
Susan Redline, MD, one of the study authors and a professor of sleep medicine at Harvard Medical School, in Boston, says that going to bed and waking up at the same time each day, avoiding alcohol and tobacco before bedtime, and other good "sleep hygiene" can help people sleep longer, and probably more deeply.
But, to maximize slow-wave sleep, she adds, "Probably the most important thing is to make sure that there is not a sleep disorder like sleep apnea or periodic leg movement that is causing disruptions."
The study had some key limitations. The researchers tracked the men's sleep on a single night, for instance, and they likewise measured blood pressure just once or twice. More research will be needed to address these shortcomings and also to rule out factors besides sleep quality (such as diet or medical conditions) that may independently contribute to hypertension, the study notes.
Nor is it clear whether habitual sleep deprivation -- an increasingly common problem -- has long-term effects on overall sleep quality and slow-wave sleep, Van Cauter says.
"It's a possibility," she says. "In the sixties, the average American was reporting sleeping 8.5 hours a night. Now most studies are showing six to seven hours. That's a huge change."