It was the kind of memo that high school students would dream of getting, if they dreamed in memos.

Lisa Waller, director of the high school at Dalton, a famously rigorous private school on the Upper East Side, sent a letter to parents this summer announcing that tests and papers would be staggered to make sure students did not become overloaded. January midterms would be pushed back two weeks so students would not have to study during vacation.

Across town at the Trinity School, another of Manhattan’s elite academies, the administration has formed a task force to examine workload, and the upper school, grades 9 to 12, has been trying ways to coordinate test-taking with papers, labs and other projects.

Horace Mann School, in the Bronx, opened a tutoring center this year to help students manage their work. Hunter College High School, which has a tough admissions exam, is for the first time this year offering homework holidays, on Halloween, the Chinese New Year (Jan. 23) and a day nearer spring, March 14.

Armed with neuroscience, self-analysis and common sense, some of New York City’s most competitive high schools, famed for their Marine-like mentality when it comes to homework, have begun to lighten the load for fear of crushing their teenage charges.

“We have incredibly talented high-achieving kids who need to be appropriately taken care of,” said Jessica Bagby, the head of Trinity’s upper school. “We realize the pressures on them, and to the degree that we’re complicit, we need to own that.”

Homework debates are both evergreen and charged in top-tier schools, but several private-school watchers say the recent moves to ease up are a marked shift. There remains a significant cadre of parents — call it the Tiger Mom camp — who see hard work as a rite of passage, part of what they pay $40,000 for and essential to making their children competitive. (One father commented wryly that it was unlikely that parents in India and China were fretting about overwork.)

But for the first time in recent memory, many see an edge by the other camp, fueled in part by the 2010 documentary “Race to Nowhere,” which focuses on the detrimental effects of overprogramming students who lack sleep and joy.

“There’s very little evidence that doing homework makes kids smarter,” said Adam Gopnik, an author and parent of two Dalton students. “Even if it did, there are values other than achievement. For example, let’s be curious.”

Mr. Gopnik did extensive research a few years ago when a group of Dalton parents engaged the administration in a discussion about homework loads. Broadly speaking, the research shows what logic might dictate: It is counterproductive for children to be up at 2 a.m. studying.

Dalton invited Harris Cooper, a professor of neuroscience and psychology at Duke University, to speak last spring about the link between homework and learning. “At five hours a night,” he said of the homework burden, “they likely won’t do any worse if they only bring home four.”

Teachers, parents and administrators are also beginning to look beyond the academic questions, studying research about the health effects of overscheduling and stress.

Denise Pope, a senior lecturer at the Stanford School of Education, co-authored a 2007 paper that looked at 496 students at one private and one public school and found that those with more than 3.5 hours of homework a night had an increased risk of physical and mental health issues, like sleep deprivation, ulcers and headaches. In a separate study of 26 schools, Ms. Pope said, 67 percent of more than 10,000 students reported that they were “often” or “always” stressed out.

“At some point, we say too much is too much,” Ms. Pope said. “In our study, that’s 3.5 hours.”

Not all schools, of course, are scaling back. Many administrators say the workload is critical preparation for success in college and in life, and a hallmark of their histories that helps draw hundreds of applicants every year. They point to parents, some of them alumni of the schools, who see heavy backpacks and demanding assignments as important signs of the schools’ seriousness and value.

And even at some of the schools making official statements or adjusting schedules, some parents see it as lip service.

“There’s nothing wrong with homework,” said Victoria Goldman, a parent of two children who attended private New York City schools and the author of “Manhattan Family Guide to Private Schools and Selective Public Schools.”

“These are the most competitive schools in the country,” Ms. Goldman continued. “It’s reinforcing what they are learning, and they are learning at the highest level you could possibly teach middle and high school kids.”

At Horace Mann, the student newspaper had an article last year showing that the average upper-school student slept 6.5 hours a night.

At other schools, administrators said they were serious about changing the culture.

A group of faculty members, students and administrators at Dalton last year devised what Ms. Waller called a “five-week assessment rotation,” in which major tests and papers are spread out over five weeks, after puzzling over why students had some weeks that were reasonable and others packed with exams and due dates.

Trinity is taking more tentative steps. During an intense self-assessment last year, surveys of parents, faculty members, students, alumni and administrators suggested that balance needed improvement. But when students were asked, anonymously, whether the amount of homework assigned was reasonable, the responses varied: 47 percent of 11th graders disagreed or strongly disagreed, compared with 35 percent of 12th graders and less than 30 percent of ninth graders.

That range convinced Ms. Bagby, the head of the upper school, that the school had to evaluate the issue better before making any major changes. Her biggest concern was lack of sleep. Trinity began the 2010-11 school year with a sleep expert who made clear that losing sleep meant losing productivity. “I think the students thought it was a little ironic,” Ms. Bagby noted wryly.

If understanding the pressure students face is a work in progress, the message that Trinity hopes to exert a little less of it is nonetheless being widely broadcast. One mother, speaking on the condition of anonymity because her child is applying to the school, said that during a tour with prospective kindergarten parents the head of the lower school said that she knew the school had a reputation as a pressure-cooker, but that it was becoming more “humane.”

Mr. Gopnik, the Dalton parent, said: “The wind is blowing in the direction of sanity. There’s no value in stressing kids out. You are robbing them of their childhood.”