SANAA, YEMEN - After Friday prayers, about 25 protesters stood outside Sanaa University chanting for President Ali Abdullah Saleh to resign. As the clock ticked toward 1:30 p.m., one by one, they began to leave, as did a small group of people watching them.

"They've gone to chew khat," Shihab Sharabi, 21, one of the protesters, said with a sheepish smile.

Khat, a leafy narcotic, is consumed by nearly every man here. Add that to the many reasons that Yemen's protest movement has yet to gain the same momentum as the revolts in Egypt and Tunisia, say many Yemenis.

For the past three weeks, protests have rocked this impoverished Middle Eastern capital. Some were massive; others were tiny. But one thing has remained constant: Most of the protests ended before 2 p.m. That's when many Yemenis enter khat-chewing sessions in their homes or cars, practically anywhere.

"In Yemen, chewing khat is like drinking water," said Samir al-Sami, an aid worker observing the demonstration. "We can't live without it."

Among its effects, khat is said to induce euphoria, loss of appetite and sleeplessness. The World Health Organization classifies it as a drug that, if abused, can cause mild psychological dependence. It is an illegal substance in the United States.

In Yemen, researchers estimate that as much as 80 percent of the population chews khat leaves, the vast majority starting at a young age.

That confounds Aidroos Al Naqeeb, an opposition lawmaker. When asked why Yemen, inflicted with many of the problems fueling the rebellions in Egypt and Tunisia, has not experienced a similar transformation, Naqeeb cited Yemen's weak civil society and weak culture of popular protests.

Then he added, shaking his head, that "the culture of khat is also playing a role."

Khalid al-Hamri, 22, a student, said it would be pointless to protest in the afternoon. "In the morning, all the government officials are in their offices. They will hear our protests," he said. "In the afternoon, nobody will listen to us because everybody is chewing khat."

Just then, a man walked by, his left cheek bulging with khat. He glanced at the protesters and jeered: "Don't you have anything better to do? The government will put you under its shoes." Then he walked in the direction of stores selling khat.

At the shops, there were much bigger crowds, some spilling onto the street. Everyone was clutching small plastic bags containing the leaves.

Back at the university, the protesters sat on the sidewalk, clutching Yemeni flags. Sharabi vowed that if Saleh didn't step down, they would protest all day, until midnight.

"We will bring our khat here and make a revolution," he said, as another protester walked toward the khat stores.