In her first year as a full-time student, Hannah Barrionuevo wrote a book about a dog searching for its mother and crafted a second one about a talented rabbit.

"It's done," she said, thumbing through her latest work. "I just have to publish it."

She is 6.

In Hillsborough County, Fla., kindergartners have long tackled weighty assignments during full-day classes, the kind of schedule that is being embraced by schools across the country.

Almost two in three kindergartners nationwide, or 65 percent of them, are in school five to six hours a day. That percentage of full-day students has doubled since the early 1980s.

Even a decade ago, most kindergartners went for a morning or an afternoon, not both.

The academic demands of kindergarten have jumped, too, for this generation of students. As the entry point to public schools in the United States, kindergarten is increasingly seen not as a soft step into first grade, but rather as a time of substance and standards.

In Hannah's district, where kindergarten begins at age 5, the lessons cover reading, writing, math, science, history, geography, civics and economics. Hillsborough County moved to full-day kindergarten in 1980, years ahead of the norm, to help children read and write.

"The kids are ready," said Lisa Bellock, the district's kindergarten supervisor. "They really want to learn. They don't just want to be baby-sat."

Although early-education specialists acknowledge more research is needed on the long-term benefits and drawbacks of full-day kindergarten, existing studies show clear advantages.

An Education Department analysis found that children in full-day classes made greater gains in reading and math than half-day students, even after adjusting for such factors as poverty status and class size.

Full-day classes also devote more time to math, social studies and science and to specific skills, such as writing the alphabet, the study found.

At Heritage Elementary in Tampa, teacher Lotus Eckstein assigns her students to write stories and put them into bound "books" using a computer and some adult help. Another hands-on lesson lets students see which objects float in pond water, the kind of field trip that Eckstein, a 29-year teacher, said "we simply didn't have time for in a half-day program."

The move toward longer kindergarten days is partly in response to the need for more instructional time. Schools today face federal pressure to show yearly gains in reading and math starting with third-graders, who in turn need more preparation at earlier ages.

Inner-city and rural areas have the most full-day kindergarten, driven in part by federal poverty aid, which eases pressure on working parents who are not home to watch their kids.

The steady growth is particularly notable in the South, where 83 percent of kindergartners go to class full time, far more than in other parts of the country.

Over two decades, the number of states requiring school districts to offer full-day kindergarten has grown from one to nine. A 10th state, New Jersey, requires some districts to offer it. Elsewhere in the country, local districts decide what to offer, according to an analysis by the Education Commission of the States, a nonprofit that tracks policy trends.

When school began in Tampa last year, Moira Kelley knew her son Landon was ready for a full day with no nap time at age 5. He practiced his emerging vocabulary at home, surprising his mom by using "recuperate" to describe how his play soldier was dealing with injuries.

As for the demands of the schedule on young children, another parent, Paul Jackson, said, "It's called full-day, but school is over at 2 p.m." He said his son Ben, who recently finished kindergarten at Heritage, has no problem blowing off steam during playtime.

Longer kindergarten programs, in fact, tend to allow more recess and other opportunities for kids to be little kids, said Kristie Kauerz, the author of the ECS report.

Reducing the complications for parents of arranging child care was a factor in Hillsborough County's decision to launch full-day classes 25 years ago. But helping kids form skills earlier was the main reason full-day kindergarten won broad political and financial support.

Today's push for longer classes has its problems, though, according to the ECS analysis.

It found most states lack policies that define what full-day kindergarten is, how to pay for it, how to provide it for all children and how to ensure that it has high standards.

"There is a strong trend toward expanding access, which is terrific," Kauerz said. "But if we want the best outcomes for young children, we need to make sure there are safeguards."

In the classroom in Tampa, teachers keep each lesson to about 15 minutes, understanding that kindergartners work better by staying active and moving among learning stations.

The math lesson? Counting blocks, and measuring the size of red, orange and yellow fish. The English lesson? Writing stories about special moments, like making breakfast with mom.

By 12:30 p.m. one day last spring, when students in half-day programs would be heading home, Eckstein's students were engrossed in story time and a lesson about parts of the body.

And when students finished their day at 1:50 p.m., it was hard to find a tired face.

"They're all ready for it," said Babette Doutt-Nesmith, as her son, Morgan, and his classmates grabbed their backpacks after another full day. "And they love it."

Kindergarten student Anthony DiAlessandro, center with blue shirt, works through the lunch line with his classmates at Heritage Elementary in Tampa. Students, from left, Tessa Baird, Alain Gaudin and Ben Jackson work on computer lessons, part of the all-day kindergarten program at Heritage.