As far as researchers currently know, not necessarily.
It is true that your circadian rhythm affects your cardiovascular system. The hormone cortisol, which plays a role in elevating blood pressure, peaks during the first three hours after you wake up from nocturnal sleep. So does adrenaline, which increases heart rate and blood pressure. Your blood is also more prone to clotting in the morning, while fibrinolysis, the process by which your body breaks down clots, is reduced.
These physiological factors have led researchers to believe that morning physical activity, which puts further strain on the cardiovascular system, could increase the risk of suffering a heart attack. But the emphasis is on could.
“It is difficult to link the physiological observations with hard data on exercise-related myocardial infarction or sudden cardiac death because the risk of an exercise-related SCD is so small in supervised exercise studies involving cardiac patients,” wrote Dr. Greg Atkinson, a leading researcher on circadian variation in sudden cardiac death, in an email. It’s also difficult for researchers to link exercise to sudden cardiac death in every day life because of data collection uncertainties.
In other words, it’s challenging to gather enough data on heart attacks, the time of day they occurred, and whether they occurred during exercise, to definitively say morning exercise will increase your risk of heart attack. There are some stats, however, that might ease your concern.
A study published in January in the journal Canadian Family Physician looked at the cardiovascular risks of physical activity. Researchers wrote that only four to seven percent of heart attacks in men are linked to physical activity, regardless of the time of day the exercise was performed. The rates for women, researchers noted, are even lower.
“Based on the pathophysiological responses to exercise from lab studies, afternoon/evening could be safer than the morning [for exercise] but there is sparse epidemiological data to support this,” wrote Atkinson. “An important point is that exercise is good for you in the long terms whatever the time of day it is undertaken.”
The bottom line: See your doctor if you have any concerns about your cardiovascular function. There is some evidence that the body may be more prone to heart attacks within the first three hours after you wake up from nocturnal sleep. However, researchers currently don’t have enough evidence to advise against morning exercise, and suggest the health benefits of exercise outweigh the risks of an early workout.