Blood pressure normally falls while we sleep. But for some people, that nighttime drop doesn’t happen, a pattern associated with a higher risk of heart and kidney problems.
New research published in PLOS ONE suggests the way the kidneys handle sodium throughout the day could be one piece of the puzzle. Using a computer model of the human body, a University of Waterloo researcher found that shifting the timing of the kidneys’ normal sodium-processing rhythm by several hours could interfere with the usual nighttime decline in blood pressure. The effect was more pronounced when the model simulated higher sodium intake or greater sensitivity to salt.
The findings do not mean that changing when you eat salty foods will improve nighttime blood pressure. The study did not test people or diets, and researchers say clinical studies would be needed to determine whether the patterns predicted by the model occur in real life.
Most people experience a drop in blood pressure during sleep, commonly known as nighttime “dipping.” Blood pressure is controlled by several interconnected systems, including the kidneys, blood vessels, nervous system and hormones, and many of those systems follow their own daily rhythms.
“Many systems that control blood pressure follow their own 24-hour rhythms, including the kidneys, blood vessels, nervous system and hormones,” said study author Dr. Anita Layton, a professor of applied mathematics at the University of Waterloo and Canada 150 Research Chair Laureate in Mathematical Biology and Medicine. “What we found is that the timing of these systems’ daily rhythm may be just as important as the rhythm itself.”
To investigate those interactions, Layton developed a computer model that simulated several systems involved in long-term blood pressure regulation. That allowed her to virtually change individual biological rhythms and observe what happened to blood pressure and sodium excretion over the course of a day.
The model suggested that blood vessels and the normal changes associated with sleeping and waking were major drivers of the nighttime blood pressure dip. The kidneys’ daily sodium-processing rhythm, meanwhile, played a larger role in determining when sodium was excreted.
Simply weakening the kidneys’ sodium rhythm had only a modest effect on nighttime blood pressure. Changing its timing produced a much bigger difference.
When the model delayed the kidneys’ sodium-processing cycle, more sodium was excreted at night and the simulated blood pressure shifted from a normal dipping pattern to a non-dipping pattern. In other words, the model suggests that the different systems regulating blood pressure may work best when their daily schedules remain coordinated.
Salt also appeared to matter. Higher sodium intake and greater salt sensitivity made the simulated system more vulnerable to the effects of the delayed kidney rhythm.
That finding adds another layer to scientists’ understanding of the relationship between sodium and blood pressure. Rather than suggesting that the time of day someone eats salt determines what happens to their blood pressure overnight, the study raises a more basic question: Could the timing of the body’s own sodium regulation help explain why nighttime blood pressure behaves differently in some people?
For now, the answer remains theoretical.
“This is a modelling study, so it does not yet tell people to change when they eat salt or take blood-pressure medication,” Layton said. “But it identifies a mechanism that can now be tested experimentally and clinically and may ultimately help us better understand and treat nighttime hypertension and non-dipping blood pressure.”
The research was supported by a Natural Sciences and Engineering Research Council of Canada Discovery Award.