So he did a study. He recruited 186 men and women, average age 66, who already had some damage to their heart, either through years of sustained high blood pressure or as a result of heart attack or even an infection of the heart itself. They each filled out a standard questionnaire to rate how grateful they felt for the people, places or things in their lives.

It turned out the more grateful people were, the healthier they were. "They had less depressed mood, slept better and had more energy," says Mills.

And when Mills did blood tests to measure inflammation, the body's natural response to injury, or plaque buildup in the arteries, he found lower levels among those who were grateful — an indication of better heart health.

So Mills did a small follow-up study to look even more closely at gratitude. He tested 40 patients for heart disease and noted biological indications of heart disease such as inflammation and heart rhythm. Then he asked half of the patients to keep a journal most days of the week, and write about two or three things they were grateful for. People wrote about everything, from appreciating children to being grateful for spouses, friends, pets, travel, jobs and even good food.

After two months, Mills retested all 40 patients and found health benefits for the patients who wrote in their journals. Inflammation levels were reduced, and heart rhythm improved. And when he compared their heart disease risk before and after journal writing, there was a decrease in risk after two months of writing in their journals. Those results have been submitted to a journal, but aren't yet published.

Mills isn't sure exactly how gratitude helps the heart, but he thinks it's because it reduces stress, a huge factor in heart disease.

"Taking the time to focus on what you are thankful for," he says, "letting that sense of gratitude wash over you — this helps us manage and cope."

And helps keep our hearts healthy.