Laughter may be the best medicine, but even looking forward to having a good laugh can boost the immune system and reduce stress, U.S. researchers reported on Wednesday.

Just anticipating a happy, funny event can raise levels of endorphins and other pleasure and relaxation-inducing hormones² and lower production of stress hormones, a team at the University of California Irvine said.

This team tested 16 men who all agreed they thought a certain videotape was funny. Half of them were told three days in advance they would watch it.

Those who knew in advance they would see the video started experiencing biological changes right away.

When the men watched the video, levels of cortisol, a stress hormone¹, fell 39 percent. Epinephrine fell 70 percent, while levels of the feel-good hormone endorphin rose 27 percent and growth hormone levels rose by 87 percent. "Growth hormone is very beneficial to the immune system," according to the team.

This all suggests that anticipation³ of a funny event can lower stress and stimulate⁴ the immune system, Lee Berk，an assistant professor of family medicine and researcher in complementary and alternative medicine who led the study said.

Berk and others had already shown that actually watching a funny video, or just laughing at a joke, could make healthful changes in the levels of hormones involved in stress and lower blood pressure.

"You have been thinking about it all day, so you experience a change in biology even before you get there," Berk said. "That is therapeutic⁵."

Berk said the finding strengthens the advice that everyone lighten up a little to live longer. "Anticipation is half or two-thirds the fun," he said.