CIRCADIAN MEDICINE

Circadian Rhythms in the Heart

 Circadian rhythms are important for the health and functioning of the cardiovascular system, so it is not surprising that disruption to these rhythms leads to cardiovascular diseases such as heart attacks. Various aspects of our cardiovascular physiology and even disease states show diurnal rhythms which are 24 hour changes. In most mammals, except for the nocturnal species, heart rate and blood pressure increase in the morning, peak around midday and drop at night. So it's no surprise that adverse cardiovascular events such as Myocardial Infarctions (MIs; heart attacks) also follow these patterns of occurrence. Many epidemiological studies show that heart attacks tend to have an early onset (between 6 am and noon) which corresponds to the time when blood pressure and heart rate are at their peak.  These observations play an important role in the progression of heart failure and development of sudden cardiac death.

 Epidemiological studies show that heart attacks tend to have an early onset  which corresponds to the time when blood pressure and heart rate are at their peak

 Since we've already established that our circadian rhythms are vital to the health of our cardiovascular system, now we can look at what happens when they are disrupted.  With the development of electricity and advancement of technology, it has become increasingly difficult to mimic the light/dark cycles that were once employed by our ancestors. Most people living in our modern world are exposed to light at night which can easily disrupt our circadian rhythms.  

This has consequences for our cardiovascular system. For example, when someone has a heart attack, their immune system is activated. You can think of it as, when a house goes up in flames, the alarms sound and alert firefighters who then come to the scene and help to put out the fire. However, if the firefighters are in a different time zone from the house, they may never get there on time and even if they do get there, it might be too late. Same goes for our heart. Normally, when a heart attack occurs, it sends a signal to the immune system to send its best fighters (immune cells) to promote healing of the damaged tissue. But like many physiological processes in the body circadian rhythms control the immune system. And so disruption to our rhythms makes the immune system less efficient at sending immune cells which can not only hinder the recovery of the heart, but can also cause the heart to get larger. This might seem like a good thing, but as the heart enlarges, it loses its elasticity as well as it's musculature which makes it weaker. A weaker heart is unable to properly pump blood to the rest of the body and this is what doctors are scientists refer to as heart failure. 

Although the circadian system is an important factor in the development of heart disease, it is not the be all and end all of cardiovascular disease. However, these discoveries are important, because many people are subject to the demands of the modern society, including shift working. For example, the very people we rely on for health and safety such as healthcare workers and police people are shift workers. This has a very pronounced effect on their circadian system: eating at odd hours, working and being expected to stay alert during normal sleep times etc. These effects are not only limited to shift workers but also to anyone living in our modern society. Something as simple as using our electronic devices at night can cause sleep disturbances or disrupt the careful homeostasis of our circadian rhythms. Many of the effects of our society are inevitable, but it is important to understand how our daily activities affect our cardiovascular health.
 
References:

http://circres.ahajournals.org/content/114/11/1713.short
https://www.ncbi.nlm.nih.gov/pubmed/26144940