According the a study in the American Heart Association journal, Circulation, cardiac arrest is just as likely to happen in the early evening.

"We know that when you wake in the a.m., your batteries get an extra charge; your body produces adrenaline; your body gets shot of cortisone to rev up the engines," said Dr. Basil Margolis of St. Joseph's Hospital. "It's these added chemicals which act as stimulants to the heart."

The study's researchers speculate the peak heart attack time during 5 p.m. and 8 p.m. could be triggered by a person's activity. Yet it is still unclear what people are doing in the early evening that might bring on a heart attack.

"If it's not chemical, then maybe it's situational," Margolis said. "For example, at the end of the day, are you more stressed -- you had a hard day at the office."

Researchers also said that by the end of the day, many people have had alcohol, several cups of coffee or a few cigarettes, all risk factors that can increase the risk for cardiac arrest.