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Nov. 2 (UPI) -- People who have high blood pressure at night are at increased risk for heart disease, even if their blood pressure is within normal ranges during the day, according to a study published Monday by the journal Circulation. A nighttime systolic blood pressure -- the "top" number -- that is 20 millimeters of mercury -- or mm. Hg, the unit of measure for blood pressure -- above daytime readings raises a person's risk for heart disease by 18%, the data showed.
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That same rise in nighttime blood pressure also increases a person's risk for heart failure by 25%, the researchers said.
"Nighttime blood pressure is increasingly being recognized as a predictor of cardiovascular risk," study co-author Dr. Kazuomi Kario said in a statement.
"This study provides much more in-depth information about the cardiovascular risk associated with high nighttime blood pressure," said Kario, a professor of cardiovascular medicine at Jichi Medical University in Japan.
Nearly half of all adults in the United States -- or 108 million people -- have high blood pressure, the Centers for Disease Control and Prevention estimates.
Research suggests that up to 40% of people experience rises in systolic blood pressure at night, whether or not their blood pressure is considered normal or healthy -- between 90 mm. Hg and 120 mm. Hg -- during the day.
For this study, researchers measured daytime and nighttime systolic blood pressure in 6,359 adults from across Japan between 2009 and 2017, using an at-home, wearable, ambulatory monitor.
Blood pressure was recorded during daily activities and sleep for at least 24-hours at a time, and device data were periodically downloaded at a healthcare clinic, the researchers said.
Nearly half of the study participants were male, and more than half were aged 65 years and older, according to the researchers.
All of the study participants had at least one risk factor for heart disease -- although none had been diagnosed with it -- and 75% of them were taking blood pressure medications when the study began, the researchers said.
The study participants were instructed to rest or sleep during nighttime hours and maintain their usual daytime activities, and they recorded their daily activities and sleep and wake times in a diary.
Nearly every participant recorded 20 daytime and seven nighttime automated blood pressure measurements.
By the end of the study period, participants experienced a total of 306 cardiovascular events, including 119 strokes, 99 diagnoses of coronary artery disease and 88 diagnoses of heart failure.
Those with a disrupted circadian blood pressure rhythm -- or higher blood pressure at night than during the day -- had a 48% higher risk for heart disease and were nearly three times as likely to experience heart failure, the data showed.
Circadian rhythms are the body's natural, internal process that regulates a person's sleep-wake cycle and repeats with each rotation of the Earth, or roughly every 24 hours, according to the American Heart Association.
Blood pressure typically fluctuates with a pattern that follows the circadian rhythms, with a peak in the early morning hours and a low during sleep, the researchers said.
"[Our] results indicate that nighttime systolic blood pressure was a significant, independent risk factor for cardiovascular events," Kario said.
"The study highlights the importance of including nighttime blood pressure monitoring in patient management strategies and will hopefully encourage physicians to ensure that anti-hypertensive therapy is effectively lowering blood pressure throughout the 24-hour dosing period," he said.
