On a typical day at Harvard, 6500 undergraduates wander through the Yard, filling lecture halls and libraries in pursuit of their sacred degrees.

But as soon as the sun sets, more than 10,000 New Englanders replace the University's traditional students, invading the same classrooms for the nighttime edition of an Ivy League education. They come from as far away as Maine and Vermont and range in age from the early teens to the mid-eighties. And most of them aren't even vying for degrees.

They are Extension School students, and their presence is becoming increasingly familiar all around campus. During the past decade alone, the number of students in the program has more than doubled. By the time the school celebrated its 75th anniversary last year, Harvard's extension had already educated more than a quarter of a million people.

The cost for continuing adult education classes varies depending upon the course and the number of credits. A non-credit liberal arts course, for example, runs for only $120, while a workshop for graduate credit lists for $500.

In all, the Extension School generates almost $1 million in income for the University--more than any other tuition-receiving school at Harvard--according to figures for the fiscal year ending last June. Without profits from the money-making program, the University would have reported a deficit of $323,000 in 1985.

Most Not Seeking Degrees

The reasons people give for enrolling in Extension School courses are as varied as the students themselves.

Some, of course, hope to earn a Harvard degree. The program offers students associates, bachelors, and masters degrees, or allows them to graduate with Extension School certificates.

Take William S. Willer, 21, of Newton, who works at Boston Five Savings Bank, and attends the Extention School in hopes of earning a bachelor degree in Economics.

"It should take me a year and a half to get the credits," Willer says. "After that, I'd like to go to business school full time. Many schools look favorably on people who work and attend school at the same time."

But students like Willer, who are seeking degrees, constitute the minority, says Michael Shinagel, dean of the University's education outreach program. Less than 2000 out of the more than 250,000 who have taken Extension courses have actually earned college degrees, the school's dean says. "That comes out to less than one percent."

Most students, like 39-year old Elaine C. Page, enroll to brush up on subjects related to their profession. A resident of nearby Bolton, Massachusetts, Page is a software editor who took "Intensive Grammar Review" last term.

"My job involves rewording software," she says. "And I had forgotten some of the rules which apply to what I do. When you change software around, you have to be able to back up what you do."

Page says that the idea of enrolling was strictly her own. "My boss did not tell me to take this course. I just thought it would help me in my job."

Susan M. Hanley, 29, of Cambridge, is a research associate in a genetics research company, BioTechnica International, working in yeast mold biology. While she was an undergraduate, Hanley majored in chemistry, but has spent the last seven years researching E. coli, a common bacterium.