Most people's blood pressure falls while they sleep, giving the cardiovascular system a period of relative rest. But for some people, blood pressure remains elevated overnight, a pattern that has been linked to a higher risk of cardiovascular and kidney problems.

New research suggests the kidneys could hold part of the explanation.

Researchers at the University of Waterloo used a computer model of the human body to investigate how the timing of kidney activity affects blood pressure throughout the day and night. Their findings suggest that even a small disruption in the kidney's daily rhythm may prevent blood pressure from falling normally during sleep.

What Happens to Blood Pressure During Sleep?

Blood pressure typically falls by around 10% to 20% during sleep. This pattern is known as normal dipping.

However, some people experience a smaller decline of less than 10%, known as 'non-dipping'. In some cases, blood pressure can actually rise overnight, a pattern sometimes called 'reverse dipping'. Both patterns can be associated with increased cardiovascular risk.

Night-time blood pressure is important because it may provide information that a daytime reading does not capture. The European Society of Hypertension describes night-time blood pressure as an established predictor of cardiovascular events.

How Could the Kidneys Affect Night-Time Blood Pressure?

The kidneys help regulate blood pressure partly by controlling how much sodium and water the body retains or excretes.

This process is not completely constant throughout the day. Kidney function, including sodium and water handling, follows daily biological rhythms that interact with hormones, the nervous system and blood vessels.

The new modelling study suggests that the timing of these kidney rhythms may matter as well.

Researchers found that weakening the kidney's daily rhythm had only a modest effect on night-time blood pressure. But when the rhythm was delayed by several hours, the model could shift a healthy blood-pressure pattern towards a non-dipping pattern. The effect was stronger when the model represented higher salt intake or greater salt sensitivity.

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Does This Mean You Should Cut Salt at Night?

The findings do not show that changing when you eat salt will lower your blood pressure. They also do not provide evidence that people should change the timing of their blood-pressure medication.

The researchers stressed that this was a modelling study, meaning the findings need to be tested in laboratory and clinical studies before they can translate into specific recommendations.

However, the research adds to evidence that blood pressure is influenced by more than how much salt a person consumes. The body's internal clocks and the timing of kidney, hormonal, nervous-system, and cardiovascular activity may also play a role.

Should You Be Concerned About Night-Time Blood Pressure?

A person cannot reliably determine their night-time blood pressure from how they feel. High blood pressure often causes no obvious symptoms, which is why monitoring is important.

If a healthcare professional suspects nocturnal hypertension or a non-dipping pattern, 24-hour ambulatory blood-pressure monitoring can record readings during both waking and sleeping hours. Current European guidelines define nocturnal hypertension as a sleeping systolic blood pressure above 120 mmHg and/or diastolic blood pressure above 70 mmHg when measured using ambulatory monitoring.

The new findings do not mean that everyone's blood pressure should be checked overnight. Instead, they highlight why understanding blood-pressure patterns across the full 24-hour cycle could be important, particularly for people already at risk of hypertension, cardiovascular disease or kidney disease.

Frequently Asked Questions

  • What is the normal pattern of blood pressure during sleep?
    Blood pressure typically falls by around 10% to 20% during sleep, a pattern known as normal dipping.
  • What is non-dipping in blood pressure?
    Non-dipping refers to a smaller decline in blood pressure of less than 10% during sleep.
  • How do kidneys affect night-time blood pressure?
    Kidneys help regulate blood pressure by controlling sodium and water retention, and their function follows daily biological rhythms.
  • Should you change your salt intake at night based on this study?
    The study does not provide evidence that changing when you eat salt will lower your blood pressure.
  • How can night-time blood pressure be monitored?
    24-hour ambulatory blood-pressure monitoring can record readings during both waking and sleeping hours.
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