In the study, men with the lowest level of the deeper stages of slumber, known as slow-wave sleep, had an 80% higher chance of developing high blood pressure than men with the highest level of this restorative sleep.
The link held regardless of other factors, such as obesity or how long the men slept.
"Reductions in the deepest stage of sleep is specifically associated with an increased risk of developing high blood pressure," said Dr Susan Redline, the Peter C. Farrell Professor of Sleep Medicine at Brigham & Women's Hospital, Beth Israel Deaconess Medical Center and Harvard Medical School.
The study, published in the journal Hypertension, reinforces other research that has linked sleep problems with a raised risk of obesity and cardiovascular problems, among other ills.
High BP and sleeping problems
Redline evaluated 784 men, average age 75, who were part of the Outcomes of Sleep Disorders in Older Men Study. In 2003-05, the men did not have high blood pressure. Ideally, blood pressure readings should be below 120/80. When they returned for a follow up in 2007-09, the investigators found that 243 men had developed high blood pressure.
The researchers divided the men into four groups, from those with the lowest amount of slow-wave sleep to the highest.
After the researchers took into account age, race, body mass index and other factors, the link between low slow-wave sleep and higher blood pressure held. Even when the researchers took into account sleep-disordered breathing and the length of overall sleep, the link held.
Slow-wave sleep decreases with age, Redline said. "Kids may have 40% slow-wave sleep [of total sleep]," she said, but healthy adults, overall, may have only about 25%.
In this study, the men averaged 11.2% of slow-wave sleep, she said. Those in the lowest of the four groups averaged only 4% or less. Other studies have shown that slow-wave sleep is lower in older men than in women.
While the study found an association between slow-wave sleep and high blood pressure, it did not prove a cause-and-effect.
The new research adds to information about the importance of sleep, said Dr. Alberto Ramos, co-director of the University of Miami Sleep Disorders Center.
Quality of sleep is important
Experts agree, he said, that not enough sleep, over time, can boost the risk of high blood pressure.
The new study, he noted, goes further by suggesting that "the quality of your sleep, specifically the amount of deep sleep, is as important in your overall health - or at least in developing high blood pressure - as the actual amount of time you sleep."
Exactly why this is so is not known for sure, according to Redline and Ramos.
"When you go to sleep, blood pressure normally falls," Redline said. "A lot of that fall occurs during slow-wave sleep." When that typical drop in night-time pressure is affected by a lack of deep sleep, "it may set you up for daytime elevations," she added.
Ramos agreed that might explain the link.
Adults can take measures to try to increase slow-wave sleep, Redline said. "Anything that wakes you up repeatedly will disrupt your slow-wave sleep," she noted.
Getting treated for underlying disorders that can wake you up - such as sleep apnoea - could help, she said.
The take-home message, Ramos said, is to pay attention to sleep quality, not just the length of your sleeping hours. Even though the link between low deep sleep and high blood pressure held after taking weight into account, Ramos noticed those in the lowest deep-sleep groups also tended to be heavier than the others. Avoiding obesity would be a good idea, he said.
To learn more about sleep, visit the American Academy of Sleep Medicine.
SOURCES: Susan Redline, M.D., Peter C. Farrell Professor of Sleep Medicine, Brigham and Women's Hospital, Beth Israel Deaconess Medical Center and Harvard Medical School, Boston; Alberto Ramos, M.D., assistant professor, neurology, and co-director, Sleep Disorders Center, University of Miami Miller School of Medicine; Aug. 29, 2011, Hypertension, online
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