Artery calcification is almost twice as common in night owls compared to early birds, according to a study from the University of Gothenburg, Sweden. Circadian function appears to be particularly important during the early stages of cardiovascular disease. 

Atherosclerosis involves fatty deposits accumulating on the inside of the arteries, making it harder for blood to pass through. The disease develops over a very long period of time and is not noticed until it leads to blood clots causing angina, heart attack, or stroke. Previous research has shown that people with late-night habits have an increased risk of cardiovascular disease, but this is the first study to show how circadian rhythms specifically affect calcification of the arteries. 

Coronary artery calcification 

The study, which has been published in the journal Sleep Medicine, involved 771 men and women aged between 50 and 64, all of whom are part of the larger population study SCAPIS. The degree of artery calcification in the heart’s coronary arteries was examined using computer tomography. Participants themselves indicated their so called chronotype on a five-point scale: extreme morning type, moderate morning type, intermediate type, moderate evening type, or extreme evening type.  

Of the 771 participants, 144 identified as extreme morning types, and 128 as extreme evening types. Among the group who were most alert in the morning, 22.2% had pronounced artery calcification – the lowest proportion of all five chronotypes. The extreme evening type group had the highest prevalence of severe coronary artery calcification, at 40.6%. 

The first author of the study is Mio Kobayashi Frisk, a doctoral student at Sahlgrenska Academy, University of Gothenburg:  

“Our results indicate that extreme evening chronotype may be linked not only to poorer cardiovascular health in general, but also more specifically to calcification in the coronary arteries calcification and atherosclerosis,” Mio Kobayashi Frisk says.   

Preventive treatment 

The statistical analysis considered a range of other factors that can affect the risk of atherosclerosis, including blood pressure, blood lipids, weight, physical activity, stress level, sleep, and smoking.  

The last author of the study is Ding Zou, a researcher at Sahlgrenska Academy, University of Gothenburg: 

“As well as the previously known factors, the individual circadian rhythm also appears to be an important risk factor for atherosclerosis. We interpret our results as indicating that circadian rhythms are more significant early in the disease process. It should therefore particularly be considered in the preventive treatment of cardiovascular diseases”, says Ding Zou. 

Self-reported chronotype 

Those who had experienced a heart attack were excluded from the study, meaning that the study participants were healthier than the general population. Another weakness identified by the researchers is that participants themselves provided their chronotype.  

Each chronotype can be said to have an average time when half of the night’s sleep has passed. In a previous study on the same population, though not necessarily the same individuals, this time occurred at 02:55 AM for the extreme morning type group and at 04:25 AM for the extreme evening type group. With the remaining chronotype groups’ mid-sleep times were somewhere in between these extremes. 

SCAPIS stands for Swedish CArdioPulmonary bioImage Study. It is a research project within the field of cardiac, vascular, and pulmonary disease. As part of the project, 30,000 randomly selected Swedes aged 50–64 have undergone extensive health examinations: blood sampling, functional tests, and radiological advanced imaging of organs and blood vessels. Six universities and university hospitals are leading and running SCAPIS in close collaboration with the Swedish Heart-Lung Foundation, the study’s main funder.

IMAGE CREDIT: Pragyan Bezbaruah.

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