Men who get the least deep sleep each night have a higher risk of hypertension, new research shows.
Earlier studies have tied chronic sleep disorders and low levels of sleep to greater risks of heart disease and obesity, and even reduced life span. But the new study,published in the journal Hypertension, is one of the first to find that it’s not just how much you sleep, but the the quality of your nightly slumber that can affect your risk for high blood pressure.
The goal of the study, carried out by researchers at Harvard Medical School and elsewhere, was to look specifically at the slow-wave stages of sleep, which make up about 90 minutes to two hours of a normal night’s rest and represent the deepest hours of sleep. To study the effect of deep sleep on health, the scientists followed 784 healthy men who were part of an ongoing sleep study and did not have signs of high blood pressure at the start of the research. During the three-and-a-half year study, the men had their blood pressure checked at various times, and their levels of slow-wave sleep were monitored at home by a machine.
After controlling for a number of variables, the researchers found that the men who spent the least time in slow-wave or deep sleep were the most likely to develop high blood pressure. Although a night of normal sleep should consist of about 25 percent slow-wave sleep, the men in the study who were at highest risk for hypertension managed to enjoy deep sleep for no more than 4 percent of their total sleep each night.
The researchers found that the men with the least deep sleep were more likely to have sleep apnea and tended to sleep less over all.
“But those conditions did not completely explain their lack of slow-wave sleep,” said Dr. Susan Redline, an author of the study and a professor of sleep medicine at Harvard Medical School and Brigham and Women’s hospital.
Dr. Redline said that although the study followed only men, she believes the results would also apply to women who fail to get enough deep sleep.
During slow-wave sleep, the brain’s electrical activity slows down, as do a person’s heart rate, adrenaline levels and blood pressure. The average person’s blood pressure falls about 10 millimeters of mercury during slumber, a dip that largely occurs when deep sleep sets in.
This nightly fall in blood pressure is a “good thing,” said Dr. Redline. When blood pressure doesn’t fall at night, the condition is called “nondipping,” and it’s a risk factor for heart disease.
“Not having that nightly dip might directly influence daytime blood pressure,” said Dr. Redline.
It’s also known that the areas of the brain that regulate sleep patterns have “a lot of crosstalk with areas of the brain that release hormones and other mediators that influence blood pressure,” Dr. Redline said. When those areas of the brain are not entering slow-wave sleep, she added, it may interfere with various brain signals that influence blood pressure.
Whether you get an adequate amount of slow-wave sleep can be influenced by a number of factors. Any condition that disrupts your sleep at night — for example, loud snoring, sleep apnea or restless legs syndrome — can shorten your slow-wave sleep, as can medications. Even your age can have an effect: Deep sleep declines as you get older.
There is also data showing that what you do during the day can make a difference. Studies show that being more physically and cognitively active can increase the amount of time you spend in deep sleep at night.
“Some of that research comes from animals,” Dr. Redline said. “If you give animals more tasks to do during the day they have more slow-wave sleep at night.”
The only way to know precisely the amount of time you drift into deep sleep each night is through an overnight sleep study. But the key, Dr. Redline said, is to assess whether you feel rested in the morning and alert and ready to go after seven to eight hours of sleep.
“If you’re feeling tired and unrefreshed after a full night’s sleep,” she said, “that’s a good indication you need to talk to your doctor, and then your doctor would decide whether you need to see a sleep specialist.”