Circadian Rhythms and Heart Attacks
Author: C. Michael Wright, MD
We all know it can be stressful to transition from sleep to wake. Some mornings are worse than others. The alarm rings, we turn it off, and experience an overwhelming desire to go back to sleep. That transition from sleep state to wake state is governed by circadian rhythms. The biological clocks in our cells are shifting cellular function from reparative and restorative modes to active operational modes needed to send us out into the world for another day of survival challenges.
For someone who is at high risk for a heart attack, the chances the event will occur in the waking hours are about 40% higher than at other times of the day. First of all, the most rapid rise in blood pressure occurs in the early morning, along with a quickening of the pulse. This is due to a rise in the hormones, such as Cortisol, and nerve signals that control cardiac contractility and cardiac output. Circadian clocks also cause a rise in clotting factors in the early morning. Higher blood pressure and faster pulse mean the heart is working harder, and therefore will be more sensitive to perturbations in oxygen delivery. Heart attacks are typically caused by a plaque rupture in a coronary artery. The rupture triggers a clot to form to seal the artery wall. In the early morning, that clot will be larger and more dangerous.
Researchers have shown in a mouse model how time of day affects the size of a heart attack. By restricting blood flow to a part of the heart for a defined period of time and then reopening the artery, one can measure the ability of the heart to recover from the damage. When arteries were occluded at the time of the sleep/wake transition, the size of the heart attack was 3.5 times larger, compared to other times of the day/night cycle. It seems that the mouse heart cells were less able to martial internal energy stores to resist the lack of oxygen when they were stressed at this critical waking time.
Other studies have shown that the potassium channel in the heart’s conduction system fluctuates to a circadian pattern, with varying amounts of potassium flowing in and out depending on the time of day. This potassium current has a profound effect on the time it takes the heart to repolarize (go back to a resting state) after a depolarization (electrical wave that triggers cardiac contraction). This shows up on the electrocardiogram as a change in what’s called the QT interval. It’s well known that QT shortening or prolongation makes the heart more prone to lethal arrhythmias.
To show just how exquisitely the heart is tuned by biological clocks, researchers in Sweden studied the number of heart attacks that occurred both the week before and the week after the start of daylight savings time. There was a statistically significant 5% increase in heart attacks in the days after turning the clock ahead. This is because we have artificially shifted our body’s natural rhythm controlled by sunrise and forced ourselves to wake up an hour earlier than expected. A potential solution for people who know they are at higher risk would be to gradually shift time forward, say 15 minutes a day over four days, starting two days before daylight savings starts.
So if you believe you are at increased risk for a heart attack, be sure to reduce circadian clock disruptions by getting enough sleep, establishing a predictable sleep/wake cycle, and following a 12 hour or less temporal feeding window, preferably cutting way back on calories, caffeine, and alcohol several hours before turning the lights off.