Why does the heart slow down at night?

A 90-year consensus on mechanisms for regulating the day-night rhythm of heart rate has been fundamentally challenged by an international team of scientists. The vagus nerve, one of the autonomic nerves that supplies internal organs including the heart, has long been believed to be responsible for slowing heart rate at night. A study by researchers from the University of Manchester, in collaboration with colleagues from London, Milan, Maastricht, Trondheim and Montpellier, found that the vagus nerve is unlikely to be directly involved. Instead, the sinus node, the heart’s natural pacemaker, has its own biological clock and knows when it’s dark and slows the heart rate accordingly.

A cycardic rhythm in the heart pacemaker

Changes in the HCN4 ion channel, a key protein that controls heart rate, at different times of the day and night may explain the changes in heart rate. The team of researchers found that blocking this channel with ivabradine, a treatment for angina pectoris, removes the difference in heart rate between day and night. The BMAL1 clock gene acts as a regulator of this channel and targeting it could lead to the treatment of bradyarrhythmias, heart rhythm disturbances, which are dangerous when we sleep.

“For the first time, we tested an alternative hypothesis that there is a circadian rhythm in the heart’s intrinsic pacemaker – the sinus node., says Alicia D’Souza, colleague of the British Heart Foundation at the University of Manchester and author of the study published December 2 in the journal Heart rhythm. Our study shows that this is the case in mice and this explains why the heart rate is slower at night. These basic mechanisms of heart rate regulation are preserved in mammals, including humans, and thus the widely accepted concepts taught in schools may one day need to be revised.. ‘Although the research was conducted in mice and rats, ion channels and clock genes play similar roles in humans.

90 years of wrong beliefs

The sinus node, sometimes called the sinoatrial node, generates electrical impulses that make the heart beat. It consists of a group of cells in the upper right chamber of the heart. The study authors used a series of measurements to assess the electrical activity and genes of the pacemaker. These included the study of heart rate and activity levels, as well as further exploration of ion currents, proteins and regulatory proteins called transcription factors.

These findings question decades of studies based on the variability of heart rate introduced by the vagus nerve. “Human resting heart rate is known to vary over 24 hours and is higher during the day than at night. But for over 90 years, daily changes in our heart rate have been – and we find too simplistic – presumed to be the result of a more active vagus nerve at night. In the future, these findings may have significant therapeutic potential in how we can understand and treat heart rhythm disorders.“Concluded Cali Anderson, author of the study.