The study, sponsored by the National Institute of Health's National Heart, Lung, and Blood Institute, also finds that patients with implantable cardioverter defibrillators (ICDs) did not experience increased death rates during the high-risk months.

"While this seasonal variation in mortality has been known for years, this is the first study that points to sudden cardiac death as the cause," said Stephen C. Hammill, M.D., president of the Heart Rhythm Society. "This is an important finding, as it demonstrates the vital role that ICDs play in preventing this life-threatening condition."

Researchers note that study data did not show an increase in mortality associated with birthdays, holidays or days of the week.

"We need to continue to work on identifying patients who would benefit from these implantable devices to reduce the number of sudden cardiac deaths," Dr. Hammill concluded.