Laughter really is the best medicine? In many ways, that’s no joke.

“There is no doubt about it,” he says. “Laughter is my first priority. I watch something every night that makes me laugh. I wake up and tickle myself while I’m still in bed. There is no greater pleasure than pointing at something, smiling and laughing about it. I don’t think there is anything more important than being able to laugh. When you can laugh, life is worth living. It keeps me going. It keeps me young. ”

In 2017, Reiner hosted an HBO documentary, “If You’re Not in the Obit, Eat Breakfast,” featuring a number of still-active nonagenarians, including Brooks, who will be 93 this month, Van Dyke, 93, TV producer Norman Lear, who will be 97 next month, and actress Betty White, 97. He believes their good health — and his — is why they still enjoy humor and stay funny.

“You can’t laugh unless you’re feeling good enough to laugh,” he says.

True — but experts who study the effects of humor say it works both ways. It’s easy to laugh when you are well, but studies suggest that laughter also can improve health and possibly stave off disease, thereby extending life. It also eases stress, and helps the ill cope with their sickness and pain.

“A friendly sense of humor will bless you with better social relations as well as coping skills, and the reduced risk of dying early,” says Sven Svebak, professor emeritus at the Norwegian University of Science and Technology, who has studied the health impact of humor for more than 50 years. “A friendly sense of humor acts like shock absorbers in a car, a mental shock absorber in everyday life to help us cope better with a range of frustrations, hassles and irritations.”

Norman Cousins, political journalist, author and longtime editor of the Saturday Review, popularized the healing properties of laughter in his 1979 book, “Anatomy of an Illness as Perceived by the Patient: Reflections on Healing and Regeneration.” In it, he asserted that self-induced bouts of laughter (and massive intravenous doses of vitamin C) extended his life after he was diagnosed with ankylosing spondylitis, a debilitating form of arthritis. Cousins lived many years longer than his doctors initially predicted.

To be sure, Cousins’s experience was not a rigorously controlled scientific experiment. Nevertheless, evidence suggests he might have been onto something.

“When people are funny, they attract other people, and community connectedness is the social currency for longevity,” says Edward Creagan, professor of medical oncology at the Mayo Clinic College of Medicine and Science. “Nobody wants to be around negative, whiny people. It’s a drain. We’re attracted to funny people.”

That old cliche about laughter being the best medicine, as with many cliches, is probably grounded in truth. The psychological effects of laughter are obvious, but it may bring physiological benefits as well. Moreover, it’s free and has no bad side effects.

Laughter stimulates the body’s organs by increasing oxygen intake to the heart, lungs and muscles, and stimulates the brain to release more endorphins, according to the Mayo Clinic. It also helps people handle stress by easing tension, relaxing the muscles and lowering blood pressure. It relieves pain, and improves mood. Laughter also strengthens the immune system.

“When we laugh, it decreases the level of the evil stress hormone cortisol,” Creagan says. “When we are stressed, it goes high and this interferes with the parts of the brain that regulate emotions. When that happens, the immune system deteriorates and becomes washed in a sea of inflammation, which is a factor in heart disease, cancer and dementia. Cortisol interferes with the body’s immune system, putting us at risk for these three groups of diseases.”

For sick people, laughter can distract from pain and provide them with a sense of control when they otherwise might feel powerless, experts say. Moreover, it’s often the patients themselves who crack the jokes.

“Some of the funniest patients I have ever met were those dying of cancer or struggling with alcoholism,” Creagan says.

One woman with breast cancer Creagan treated for 15 years still was making jokes as she neared death. During her final visit, she asked the doctor how much time she had left.

“I asked her why this was important to her right now,” Creagan recalls. “She said: ‘I can max out all my husband’s credit cards, so there’ll be nothing left for the second wife.’ I think she got all those extra quality years because she was funny.”

Deborah Mayer, interim director of the National Cancer Institute’s Office of Cancer Survivorship, agrees that humor is best initiated by the patient, and shared with other patients.

“Some of the things they say are hysterical, but their families would be horrified,” she says. “I don’t know if humor does extend your life, but it certainly can make your life better for as long as you live it.”

The results of a large, 15-year Norwegian study of 53,556 participants conducted by Svebak and his colleagues indicate that humor can delay or prevent certain life-threatening diseases. The scientists measured the subjects’ sense of humor with a health survey that included, among other things, a cognitive element, “asking the participants to estimate their ability to find something funny in most situations,” Svebak says.

Women with high cognitive scores experienced a reduced risk of premature death from cardiovascular and infectious diseases. Men who scored high cognitively had a reduced risk of early death from infections. The study found no effects on cancer and other causes of death. The benefits gradually faded with increasing age and disappeared after age 85. “This means that a higher than average cognitive sense of humor is no vaccine to protect you against death in the end, although it will increase your probability of getting old,” Svebak says.

Humor also seems to stimulate memories and improve mental acuity in the elderly, especially among those with dementia. The therapeutic benefits of “clown therapy” for hospitalized pediatric patients is well-established, but elder clowns are now also helping seniors in residential settings, says Bernie Warren, professor emeritus in dramatic arts at the University of Windsor and founder of Fools for Health, a Canadian clown-doctor program.

Elder clowns visit residents as often as twice weekly and use comedy, music, storytelling and other techniques to arouse memories and responsiveness.

“They don’t wear a white coat or a stethoscope,” as clowns do with children, because “they aren’t going into a hospital where someone is sick, but entering someone’s home, which may be the last home they ever have,” Warren says.

He has seen Alzheimer’s patients engage with clowns “and become lucid and aware,” Warren says. “There’s anecdotal evidence that suggests clowns help greatly with memory, language and communication and awareness of self in the present. That red nose enables the person, somehow, to walk a metaphoric bridge of memories that had been lost to them. They go back to their childhood, collect the memory, and then bring it back to present.”

Most experts believe that the benefits of laughter accrue when people enjoy humor along with others who are like-minded.

“Your sense of humor is anchored within your personal experiences and, therefore, changes throughout your life,” Svebak says. “The safest place for sharing humor is among those with whom you share experiences.”

Reiner’s exposure to humor began in childhood when his immigrant parents took their children to see Buster Keaton, Marx Brothers and Charlie Chaplin movies, and often tuned their radio to comedy programs at home. He began his career in the 1950s writing and acting on “Caesar’s Hour” and “Your Show of Shows,” and later created, wrote and acted in “The Dick Van Dyke Show.”

More than 60 years ago, he and Brooks — who still visits Reiner at home several times a week — began performing an iconic comedy sketch at parties, which comedian Steve Allen persuaded them to record. “The 2000-Year-Old Man” features Reiner, the interviewer, probing the memories of a character, improvised by Brooks with a Yiddish accent, who claims to have lived for 2,000 years. It was an instant hit, and still is considered a classic.

At one point, Reiner, the interviewer, asks Brooks, the ancient character, to reveal the secrets of his very long life.

“The major thing, the major thing, is that I never, ever eat fried food,” the old man says. “I don’t eat it, I wouldn’t look at it and I don’t touch it. Never run for a bus, there’ll always be another.” He also warns people to stay out of small Italian sports cars, especially Ferraris, and urges them to eat lots of fruit, particularly nectarines. “Even a rotten one is good,” he says. “I’d rather eat a rotten nectarine than a fine plum. What do you think of that?”

Today, Reiner says he believes the old man should have added one more piece of advice to that list. “Keep laughing,” Reiner says. “You’ll live forever.”