Singer-songwriter Leonard Cohen announced he's started to smoke cigarettes again at the age of 80. With average male life expectancy in the US currently hovering at 76, Cohen has surpassed the mark and decided to focus on his happiness.

"If I had taken my doctor's advice and quit smoking when he advised me to, I wouldn't have lived to go to his funeral," cigar smoker George Burns famously quipped. When is it okay to ignore medical advice and just focus on being happy?

On Boston Public Radio, medical ethicist Art Caplan said his thinking runs counter to conventional medical wisdom. "I happen to agree with Leonard Cohen," Caplan said.

Caplan credited interactions he's had with patients for changing his mind on the subject. "The VA hospital here, there were vets well into their eighties that wanted to continue to smoke, and we were pushing 'stop smoking' campaigns on them," Caplan recounted. "I finally said, 'What are we doing here?'"

By starting smoking, Cohen is certainly in good company: the US smoking rate is about 18 percent — roughly 42.1 million Americans — according to the Centers for Disease Control. Those smokers face a higher risk for many types of cancer, emphysema, and heart disease.

Caplan noted Americans tend to overindulge in things like junk food. "I'm not sure Americans forego a lot of pleasures," Caplain said, "but when you get older and people say, 'Boy you better not have that second beer, or not have that second scoop of ice cream,'" that advice may go too far.

Boston Public Radiocohost Jim Braude said happiness and conventional health wisdom may be at odds. "You can occasionally slip," Braude said. He advised, "Don't wait until you're 80."

Caplan responded to Braude by explaining the paradox of exercise. Caplan cited a physiologist friend of his who said, "'For every two minutes you jog you probably add about a minute of time to your life.' That's fine, but it really makes sense to jog if you like jogging. But if you don't — you're losing time!" Caplain said.

Caplan said the life-longevity calculus has been instructive in his own life. "I could live much longer, and I'd also find out I'd have to go to faculty meetings much more," he said.

Boston Public Radio cohost Margery Eagan admitted to a recent health lapse in the name of happiness. "Last night I went out with a friend of mine, I had a big, fat, juicy cheeseburger with fries on the side," Eagan said.

Caplan cited Aristotle as the final arbiter on happiness and health: "Moderation in all things, excess in none." Nota bene, Leonard Cohen.

>> To hear more from Caplan, including discussion about putting Lithium in the drinking water, click the audio link above.