This article originally appeared in the December 2004 Harvard Men’s Health Watch and is provided courtesy of Harvard Health Publications.

Timing is everything: Fluctuations in cardiac risk

Many human activities are governed by the calendar and the clock, and an expanding body of evidence suggests that human disease also varies with time and date. It’s easy to see why heat stroke occurs in summer, frostbite in winter, and it’s natural that influenza and pneumonia peak in winter, when folks are crowded together, coughing on one another. But what about heart attacks? Spring may make the heart beat fonder, but heart attacks don’t follow the seasons, much less the clock — or do they?

Seasonal variations
T. S. Eliot believed, “April is the cruelest month.” Even with your tax bill coming due, however, it’s not the hardest month for your heart. For cardiac cruelty, try January. Heart attacks peak during the winter months, and data from the Second National Registry of Myocardial Infarction (heart attacks) make it clear. When researchers analyzed 259,891 heart attacks reported over a two-year span by 1,474 hospitals throughout the United States, they found a marked seasonal variation. Winter was the top heart attack season, followed by fall, then spring, then summer. All in all, there were 53% more heart attacks in winter (December 21–March 19) than summer (June 21–September 22). January was the leader, with twice as many heart attacks per day than July, the safest month. And in addition to being more common in the winter, heart attacks were also more serious, with a 9% fatality rate. Another study found that winter heart attacks produce more damage to cardiac muscle than those in any other season.

This nationwide American study is an impressive demonstration of the seasonal variation in heart attack risk, but it does not stand alone. Studies from Australia to Ireland and England to Romania and Bulgaria have yielded similar findings. The seasonal flux is generally greatest in temperate zones, but it also occurs in places that are warm year-round. In Hawaii, the heart attack death rate is 22% higher in winter than summer. In Los Angeles it is 30% higher, with the rise beginning around Thanksgiving and peaking around New Year’s Day.

Heart attacks are not the only cardiac problems that peak in winter. A six-year study from Scotland found that hospital admissions for congestive heart failure were 15%–18% higher in winter than during the rest of the year. Similarly, serious, potentially fatal disorders of the heart’s pumping rhythm are more common in winter than in any other season.

What accounts for the trend? Doctors don’t know for sure, but several factors are likely to contribute.

Low temperature. Cold is a vasoconstrictor. To conserve internal heat in a cold environment, the body narrows the blood vessels in the skin and limbs. It’s why your hands and feet are pale and icy during exposure to cold and why they are vulnerable to frostbite. But narrowed blood vessels also mean higher blood pressure readings, and hypertension puts a stress on the heart. Indeed, blood pressure readings tend to rise in winter. Still, the cold can’t explain everything, since January is the peak heart attack month in Honolulu as well as New York.

Lack of exercise. Habitual exercise protects the heart. In cold climates, people tend to slack off in winter. But cardiac protection shouldn’t vanish all at once, and in warm climates people may even get more exercise during winter months.

Snow shoveling. Even if a decline in regular exercise doesn’t play a major role, a seasonal burst of increased exercise does, at least in regions where Old Man Winter does his thing.

Holiday happenings. Whether or not there is snow on the ground, people usually celebrate the holidays with family feasts and numerous parties. Weight gain is common, but it’s usually not enough to tip a man into the hazardous realm of obesity. Alcohol is a double-edged sword; over the long run, low doses can reduce cardiac risk, but binge drinking can produce problems like high blood pressure and abnormal heart rhythms. High-fat dining may also be a factor. By interfering with arterial relaxation and by activating the clotting system, even a single high-fat meal can spell trouble for men with coronary artery disease. Sexual activity tends to peak around Christmas and New Year’s, but conventional sex with a familiar partner should be safe for all but the most fragile hearts.

Respiratory infections. Winter is the time when people stay in and bugs come out. A study of nearly 10,000 patients in Great Britain found that acute respiratory infections are associated with an increased risk of heart attacks. Simple colds won’t stress the heart, but influenza and pneumonia can, especially if they cause high fevers and low oxygen levels. It’s why immunizations for influenza and pneumonia are so important (see Flu shots, heart attacks, and health, box below).

Biological factors. A 2004 study reported that cholesterol levels peak in December for men and in January for women. New cardiac risk factors such as fibrinogen, C-reactive protein, and apolipoprotein B also rise in winter.

Stress and depression. The holiday season can be stressful for people with many commitments and depressing for those who are alone. The short daylight hours can also trigger seasonal affective disorder and depression. At any time of the year, stress and depression can be heartbreaking, and they certainly may compound winter’s woes.

Smoking. Cigarette sales peak around the holidays. Even secondhand smoke can trigger cardiac problems. It’s both depressing and stressful.

Daily variations
If the seasonal fluctuation in cardiac risk is a wake-up call this winter, the daily variation will be an eye-opener tomorrow morning.

For heart attacks, morning is prime time. A meta-analysis of 30 studies that included 66,635 patients found a 40% increase in risk between 6 a.m. and noon. Similarly, sudden cardiac deaths are 29% more common in the early morning than at other times of the day. Serious arrhythmias and deaths from congestive heart failure follow the same pattern. A 2004 study provided a pathological explanation for this pattern when it reported that ruptures of coronary artery plaques peak between 6 a.m. and noon.

The body has an internal clock that may help explain the striking distribution of cardiac risk. For one thing, hormone levels fluctuate in a very predictable pattern over a 24-hour cycle. The stress hormones adrenaline and cortisol peak in the early morning, as does the male hormone testosterone; in contrast, the “dark hormone” melatonin is produced mostly at night. Body temperature follows a similar rhythm; in most people, it’s about 97¾ at 5 a.m. and gradually rises to an average of 99.4¾ at 5 p.m. before falling again overnight. Blood pressure follows an opposite cycle, being highest in the morning and lowest during sleep. Finally, a 2004 study found that endothelial function is impaired in the early morning, so that blood vessels cannot widen normally when tissues need more blood.

Stress is the other major factor. The simple act of waking up seems to be stressful. Adrenaline jolts through the bloodstream. Blood pressure jumps. And platelets, the tiny blood cells that trigger the blood clots that can block cholesterol-laden arteries, become stickier and more likely to start the clotting process. All in all, it’s no wonder that heart attacks are particularly common within three hours of arising.

The morning peak in heart attacks raises an alarming possibility. For many men, morning is the most convenient time to exercise; does the stress of exercise add to the stress of waking, further boosting cardiac risk? Fortunately, the answer appears to be no. A study of cardiac rehabilitation programs in North Carolina found the same low rate of complications whether the program started at 7:30 a.m. or 3 p.m. All the patients followed a careful warm-up and cool-down routine, and so should you. Always stretch and warm up before you exercise, particularly if you plan to work out in dawn’s early light.

Weekly variations
The seasons of the year and the hours of the day depend on the laws of nature, based on the earth’s orbit around the sun and its rotation on its own axis. The days of the week, on the other hand, derive from the laws of man — and the five-day work week is the reason heart attacks peak on Monday. Serious arrhythmias follow the same pattern. Although the details vary, the Monday miseries have been reported in both men and women and in people who work as well as those who have retired. All in all, cardiac events are about 20% more frequent on Mondays than on the other days of the week. Saturdays and Sundays are the safest except for Frenchmen, who appear to be at higher risk during weekends; it’s one more example of the “French paradox.”

Stress is the likely explanation for the Monday peak in cardiac risk. Retirees may retain the responses they learned in their working years, and they are certainly susceptible to the hustle and bustle of a Monday morning.

Shakespeare’s Romans were cautioned to beware the Ides of March. If he were writing today, the Bard might well advise us to beware of Mondays in January.

Other diseases
Because they are vascular diseases, strokes and heart attacks have much in common. Like heart attacks, strokes are most common (and most serious) in the morning; more than 45% of all strokes occur between 6 a.m. and noon. Like heart attacks, strokes peak on Mondays. However, the seasonal distribution of strokes is less clear. Studies from Australia and Wisconsin fingered winter as the season of risk, but researchers in North Carolina implicated spring.

Three other diseases that are particularly prevalent in men tend to vary with the clock. Gout is likely to strike in the dark of night. The pain of kidney stones tends to begin in the wee hours of the morning, probably because urine production slows at night, at least in younger and middle-aged men. Finally, the cataclysm of a ruptured aortic aneurysm is most likely between 6 a.m. and noon, following the pattern of other vascular events.

Time out?
Time marches on. Even if you don’t set an alarm, your biological clock increases your cardiovascular vulnerability when you awaken in the morning. Even after you retire, you’re likely to retain some of Monday’s risk. And although you may enjoy a winter in Florida, you can’t entirely escape winter’s toll on the heart.

But if you can’t stop time, you can lighten its load. Stretch and warm up in the morning, especially before you exercise. Try to reduce your stress at work, and learn to pace yourself all week long. Avoid binge drinking and heavy, high-fat meals, particularly around the holidays. Keep warm in the winter and shovel snow with extreme care, if at all. Get your flu shot each year and a pneumonia vaccination every 5–10 years. Avoid tobacco in all its forms, including secondhand smoke. And listen to your body with extra attention at times of extra risk — it’s the timely thing to do.