NEW HAVEN — On Jan. 12, a few days after registration opened at Yale for Psyc 157, Psychology and the Good Life, roughly 300 people had signed up. Within three days, the figure had more than doubled. After three more days, about 1,200 students, or nearly one-fourth of Yale undergraduates, were enrolled.

The course, taught by Laurie Santos, 42, a psychology professor and the head of one of Yale’s residential colleges, tries to teach students how to lead a happier, more satisfying life in twice-weekly lectures.

“Students want to change, to be happier themselves, and to change the culture here on campus,” Dr. Santos said in an interview. “With one in four students at Yale taking it, if we see good habits, things like students showing more gratitude, procrastinating less, increasing social connections, we’re actually seeding change in the school’s culture.”

Dr. Santos speculated that Yale students are interested in the class because, in high school, they had to deprioritize their happiness to gain admission to the school, adopting harmful life habits that have led to what she called “the mental health crises we’re seeing at places like Yale.” A 2013 report by the Yale College Council found that more than half of undergraduates sought mental health care from the university during their time there.

“In reality, a lot of us are anxious, stressed, unhappy, numb,” said Alannah Maynez, 19, a freshman taking the course. “The fact that a class like this has such large interest speaks to how tired students are of numbing their emotions — both positive and negative — so they can focus on their work, the next step, the next accomplishment.”

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Students have long requested that Yale offer a course on positive psychology, according to Woo-Kyoung Ahn, director of undergraduate studies in psychology, who said she was “blown away” by Dr. Santos’s proposal for the class.

Administrators like Dr. Ahn expected significant enrollment for the class, but none anticipated it to be quite so large. Psychology and the Good Life, with 1,182 undergraduates currently enrolled, stands as the most popular course in Yale’s 316-year history. The previous record-holder — Psychology and the Law — was offered in 1992 and had about 1,050 students, according to Marvin Chun, the Yale College dean. Most large lectures at Yale don’t exceed 600.

Offering such a large class has come with challenges, from assembling lecture halls to hiring the 24 teaching fellows required. Because the psychology department lacked the resources to staff it fully, the fellows had to be drawn from places like Yale’s School of Public Health and law school. And with so many undergraduates enrolled in a single lecture, Yale’s hundreds of other classes — particularly those that conflict with Dr. Santos’s — may have seen decreased enrollment.

At the start of the semester the class was divided between a live lecture in the 844-seat Battell Chapel, a historic place of worship on campus, converted to a lecture hall, and one or two smaller auditoriums where several hundred more students watched a live stream of Dr. Santos. After several weeks, the decision was made to move the lectures to Woolsey Hall, usually the site of events like symphony performances, which can accommodate the entire class.

The course focuses both on positive psychology — the characteristics that allow humans to flourish, according to Dr. Santos — and behavioral change, or how to live by those lessons in real life. Students must take quizzes, complete a midterm exam and, as their final assessment, conduct what Dr. Santos calls a “Hack Yo’Self Project,” a personal self-improvement project.

Some students admit that they see the course as an opportunity to take a relaxed lecture with few requirements.

“I wouldn’t have known about the course if not for word of mouth, but it’s low-pressure, and maybe I’ll learn a few tricks to having a less stressful life,” said Riley Richmond, 22, a senior who enrolled in the class with several of his friends.

Charlotte Emerson, 18, a freshman in the course, says she worries some students will take advantage of the lack of accountability that comes with a lecture of this size. For example, Dr. Santos is not monitoring whether students complete weekly “rewirement” assignments, like performing acts of kindness and forming new social connections, Ms. Emerson said.

But while others might see easy credits, Dr. Santos refers to her course as the “hardest class at Yale”: To see real change in their life habits, students have to hold themselves accountable each day, she said.

She hopes that the social pressures associated with taking a lecture with friends will push students to work hard without provoking anxiety about grades. Dr. Santos has encouraged all students to enroll in the course on a pass-fail basis, tying into her argument that the things Yale undergraduates often connect with life satisfaction — a high grade, a prestigious internship, a good-paying job — do not increase happiness at all.

“Scientists didn’t realize this in the same way 10 or so years ago, that our intuitions about what will make us happy, like winning the lottery and getting a good grade — are totally wrong,” Dr. Santos said.

College courses on positive psychology have a track record of attracting scores of students. At Harvard, about 900 students enrolled in a lecture titled Positive Psychology in 2006. What distinguishes Dr. Santos’s course from the one at Harvard in 2006, she said, is that it also focuses on behavioral change.

Still, Dr. Santos says she does not plan to offer the course again. Dr. Ahn, of the psychology department, said, “Large courses can be amazing every once in a while, but it wouldn’t be fair to other courses and departments to take all of their students away.”

She added, “It causes conflict, and we can’t afford to offer this every year in terms of teaching fellows and resources.”

Dr. Santos said a multipart seminar-style series on the course material — filmed last year in her home and titled the Science of Well-Being — will soon be available for free on Coursera, an online education platform. For now, she is eager to see whether her teachings alter campus life.

“We have this moment where we can make a difference in Yale’s culture, where students feel like they are part of a movement and fighting the good fight,” she said.