BERKELEY - The idea of summers during college bring to mind for many people the sweet luxury of sleeping in late or working as a camp counselor or waitress -- anything but the intellectually stressful task of writing papers and studying for tests.

But an increasing number of UC Berkeley students aren't sprinting for the soft sands of the Pacific coast when final exams end in May.

Roughly 50 percent of all undergraduates gear up for summer school. As Cal's enrollment grows in response to the statewide surge in the college-age population, it's becoming difficult for students to get into required courses for their major during the regular school year.

Also, as graduate programs become increasingly competitive, more students are attending summer school to complete double majors and enhance their resumes.

Sitting on the sun-baked steps of Sproul Hall, junior Ruben Macias, a molecular and cell biology major, did his physics homework as students clad in flip-flops clopped past. He's also taking calculus because many of the required courses for science majors have long wait lists during the regular school year, and it takes a little pressure off his fall and spring schedule. But it means class four days a week from 8 a.m. to 3 p.m. and then studying from 5 p.m. to 3 a.m.

Summer school enrollment increased by five percent over last year, to roughly 17,330 students. And over the past five years, summer enrollment has soared 41 percent. That's partly because the state lowered summer school fees in 2001 to encourage enrollment and to cope with the college-age boom.

The university offers courses that tend to be crowded in the fall and spring.

Another factor that may be at play is this year's fee increase. University fees will soon likely go up at least 25 percent and many students may be taking summer classes to ensure they graduate on time, said Gary Penders, director of summer sessions.

"Students and their families don't have a clear idea of how far the fees will go up," he said. "When they see that uncertainty, they say 'Let's finish as soon as we can.'"

UC Berkeley is the land of high achievers. But during the summer it becomes, to a certain degree, a world awash in super students. Many students majoring in two subjects attend summer school to fit in all the courses they need.

Christina Loughrey, a fifth-year senior, is double majoring in rhetoric and interdisciplinary studies. It has enriched her education, she said. But it might also give her an edge in applying to graduate school. Cal is so huge that it's difficult to form relationships with professors and get them to write recommendations. Anything else a person can do helps, she said.

"People are doing whatever they can to make themselves look better."

The number of students graduating with double majors has increased from 6.1 percent in 1982 to 11 percent in 2002. Competitive graduate schools play a role in that, but Berkeley has also become so selective, its entering students are more academically capable and driven, said Gregg Thomson, director of the Office of Student Research.

Majoring in two subjects isn't as much of a feat for them.

Justina Louie, who just finished her freshman year, already plans to double major in business and history. Because the business courses are so competitive, she figures she has a better chance of maintaining good grades by spreading out her tough classes. So she's taking statistics this summer. She says doubling up with a business major will provide her with an array of important skills: strategic thinking, writing, presentation and public speaking.

The last time Louie had a summer free from advanced academic classes or work was between the fourth and fifth grade. Asked what she would do if she could design the perfect summer, she replied, "I'm kind of a nerd. ... I'm bored when I'm not in school."