If a patient had visited Dr. Alex Eingorn's basement office in Manhattan on a recent weekday after hours, he would have been perplexed by the ritual going on there.

Eingorn, a chiropractor by day, stood in a circle with five middle-aged people holding up imaginary birdcages. At a signal, the six simultaneously opened the cages and freed the pretend birds.

They doubled over with laughter and several imitated the make- believe birds by flapping their arms, prompting even more intense laughter.

No, these people are not crazy.

They are members of a laughter club, which meets weekly to practise a combination of routines that trigger laughter and yoga breathing, known as laughter yoga.

Dr. Madan Kataria, an Indian physician, founded the movement in 1995 and it has grown to more than 3,500 clubs worldwide as people search for innovative ways to relieve stress and have fun.

"There is so much negativity, misery and depression in this world," Kataria said. "The human spirit is going down day by day and laughter can elevate it. Once you feel good inside, you see the world differently."

Kataria said adults are not laughing nearly enough. Children laugh up to 300 or 400 times a day, while adults only laugh 15 times a day. Recent studies have also shown laughter can strengthen the immune system, reduce depression, improve self-confidence and even fend off heart attacks and strokes.

Laughing can be a great workout. Like any aerobic activity, it increases heart rate and circulation. It also exercises the lungs, facial muscles and diaphragm.

"It stimulates every organ and it's not boring like a treadmill, where you're running and running but not going anywhere," said Eingorn, 44.

"My mind is clear and I'm ready to go out and face the world in a positive way."

Eingorn trained with Kataria and started his own club in November 2003.

"Laughter is innate," said psychologist Steve Wilson.

"We laugh when we're young without being taught how to do it. People are hungry for anything that can relieve stress.

"They're worried about their kids, their job, the price of gasoline and whether there's going to be a bomb somewhere. They want relief."