Why heart rhythm disturbances are more likely to happen early in the morning

Research funded by the BHF has revealed for the first time a mechanism explaining why potentially lethal heart rhythm disturbances are more likely to occur in the morning, when people wake after a night's sleep. These disturbances, known as ventricular arrhythmias, are linked to the natural surge of the stress hormone cortisol, which peaks in our blood first thing in the morning.

In a study in mice, researchers at Imperial College London found that cortisol binds to a specific protein on the surface of heart cells. The protein moves to a different part of the cell where it influences genes which control how easily the heart cells carry the electrical signals that tell them to beat.

As the activity of the genes change, electrical impulses to the heart become less regular and more chaotic, leading to abnormal heart rhythms, or arrhythmia.

Cortisol's circadian, or day-night, rhythm is affected by sleep, with an increase in cortisol just minutes before an individual wakes up.

Professor James Leiper, BHF Associate Medical Director, said: "Ventricular arrhythmias can strike at any time and, if left untreated, can lead to a loss of consciousness, sudden cardiac arrest, and death. It is crucial we continue to investigate the causes of these arrythmias, so we can take action to prevent them."

"This intriguing study in mice reveals a possible solution to the mystery of why ventricular arrhythmias are more common in the morning. Identifying a rise in cortisol as the culprit could allow us to explore new treatment options that could reduce arrhythmias in those most at risk. Further research will be necessary to establish whether these findings are also seen in humans."

The discovery of the link also raises the prospect of new treatments in this field. Dr D'Souza and colleagues went on to show that, in mice, injection of a drug that blocks the cortisol receptor prevents the morning changes in ion channels and so also the morning vulnerability to heart rhythm disturbances.

Lead researcher Alicia D'Souza, a BHF Fellow at Imperial's National Heart and Lung Institute said: "Our hearts are effectively different organs at different times of the day. They are more vulnerable first thing in the morning because of ancient circadian rhythms, which have evolved over millions of years.

"All species have this in common, so although our study is in mice, we believe this is directly applicable to human and mammalian hearts."

This study is the latest in a series in which Dr D'Souza and Professor Mark Boyett from the University of Bradford have been exploring why the electrical activity of the heart shows an important day-night rhythm and why the heart is vulnerable to different arrhythmias at different times of the day or night.