Sooner or later, most presidents discover that their favorite subject of conversation is how hard it is to be president.

Jimmy Carter liked to remind people of the loneliness and difficulty of the office. Richard M. Nixon talked about burdens and crises and how hard he worked. George Reedy, who labored as press secretary in the shadow of Lyndon B. Johnson, warned afterward about the dangers of "an environment in which no man can live for any considerable length of time and retain his psychological balance."

President Reagan has started out with a notably different approach.

"Show me an executive who works long, overtime hours and I'll show you a bad executive," Reagan said while campaining in Texas last May 1.

This remark, on a television talk show, was a frank assertion of Reagan's belief that a chief executive accomplishes little if he busies himself with detail or exhausts himself with overwork. In the last 19 days, Reagan has put theory into practice.

Last week, for instance, Reagan arrived in the Oval Office at 8:45 a.m. on four of the five days of the working week. He was out of the office by 6 p.m. on two days and by 3 p.m. on Friday, his 70th birthday.

Reagan's printed daily schedules demonstrate what his White House aides freely acknowledge: the president is working shorter hours than did his predecessor, who prided himself on rising at 5:30 a.m. and being in the office by 7.

The new president is capable of burning the midnight oil, as he did last Tuesday, working on his television economic address. And, when he leaves the office for the White House family quarters, Reagan takes briefing papers with talking points and the next day's schedule, both of which are given him in a daily 5:30 p.m. meeting with senior advisers.

Overall, however, Reagan seems more comfortable with than consumed by the office, at least so far. He might even have agreed with Reedy's judgment that, "There is far less to the presidency, in terms of essential activity, than meets the eye."

Over the weekend, for instance, the president kept his "essential activity" to the bare minimum. On Saturday, he reviewed proposed budget cuts prepared by some of his budget working groups and went with Nancy to dine at Jean Louis' at a small party hosted by his old friend, Hollywood investor Charles Z. Wick. On Sunday, Reagan spent the day at the residence.

Reagan seems to genuinely believe that he can accomplish more by doing less. Whether he can maintain that approach to the job as the burdens and problems stack up is one of the interesting, even important questions about his presidency.

Clearly he is going to try the relaxed approach. Last week an aide was reminded of a telling incident during last year's campaign. One morning, when there was an early start after a late night, Reagan asked campaign adviser Stu Spencer what they were doing up so early. Spencer told the candidate he ought to get used to it because, once he became president, a National Security Council aide would be in the Oval Office at 7:30 every morning to brief him.

"Well, he'll have a helluva long wait," Reagan replied.

And so, he would have last week. All of Reagan's national security briefings were held at the more civilized hour of 9:15 a.m. except for Thursday, when the president and his wife attended the annual national prayer breakfast at 7:40 a.m. and Reagan was not briefed on world events until 10.

Critics who wish to question Reagan's presidential work habits might first pause over the warning Reedy expressed in his seminal book, The Twilight of the Presidency:

"The concept of the overburdened president represents one of the insidious forces which serve to separate the chief executive from the real universe of living, breathing troubled human beings. It is the basis for encouraging his most outrageous expressions, for pampering his most childish tantrums, for fostering his most arrogant actions."

By that standard, the early weeks of the Reagan presidency seemed remarkably underburdened, even enjoyable for the chief executive.

Last week he wasn't idle; far from it. The new president had a symbolically important meeting with congressional leaders on Capitol Hill. He faced his first legislative test on the bill to raise the federal debt ceiling. He gave a television address on the economy to which he attached high importance. Throughout, the White House mood was upheat and relaxed.

Reagan and his chief aides joked about his age, marveled at the resources and confusion of the White House and seemed to be enjoying the experience of executive power. The president's main reservation about his new job seems to be the lack of time he can spend outdoors.

He told aides he likes Camp Daivd, Md., where he spent last weekend, and he is already looking forward to a Feb. 19 trip to his ranch north of Santa Barbara, the first long weekend break he will take as president.

There are those at the White House who are concerned about the bad publicity that might attend the trip, which is to start one day after Reagan submits his budget-slashing program to Congress. But Reagan will trade the possibly negative publicity for a chance to ride on his California ranch when it has turned green with the winter rains.

On friday, Reagan's aides presented him with a birthday card that mocked a campaign line he repeated in his economic address Thursday night.

"Mr. President, are you really better off today than four years ago?" said the card. Inside was a comparison of last Friday's schedule and Reagan's schedule four years ago, which was depicted as "All day on the ranch and evening dinner at Chasen's."

Even for a president with Reagan's relaxed style, there is no such thing as a typical day. On Monday, he began his work, as he does most days, with a 30-minute meeting with counselor Edwin Meese III, chief of staff James A. Baker III and deputy chief of staff Michael K. Deaver. These are the three aides who can walk in on him at any moment if they feel the need.

Usually, as occurred this day, Deaver leaves the room after this frist staff meeting, and Vice President Bush and national security affairs adviser Richard V. Allen arrive to brief the president.

The Monday schedule shows another enlarged staff meeting, then a briefing preparatory to the visit of Korean President Chun Doo Hwan, which preoccupied Reagan until 3:30 p.m., when he gave a magazine interview, followed by the first of a series of informal interviews with a small group of columnists and reporters.

He met at 4:30 p.m. with Henry A. Kissinger, an event never announced by the White House, and after a concluding meeting with the senior staff trio of Meese, Baker and Deaver, departed for the residence at 6.

There were two evening events on the week's schedule, a Tuesday blacktie with the Washington Press Club, where Reagan proved a hit with one-liners, and his nationally televised speech Thursday on which the president predictably lavished more attention than any other event of the week.

Reagan read and then rewrote two drafts prepared for him, one by veteran Nixon speechwriters Ken Khachigian and Tony Dolan, and the other by David Gergen, a former speechwriter for Gerald R. Ford. The president told Deaver that he had awakened in the middle of the night Tuesday thinking about how to express his ideas in the simplest terms.

"I want to give the blue-collar workers out there an economics lesson," Reagan said at one point.

Reagan spent most of Thursday afternoon reworking the speech, with typists standing by to prepare the message from his handwritten drafts. Murray Weidenbaum, the new chairman of the Council of Economic Advisers, was brought in to check the facts on the final version.

Then Reagan and his aides discussed the props for the speech, with the president proposing that he use a silver dollar to illustrate the decline in purchasing power. His aides persuaded him to use a dollar bill instead.

As governor of California and as candidate for president, Reagan spent much of his time with old friends and allies, and that was true again last week. He had lunch with his former national campaign chairman, Sen. Paul D. Laxalt (R-Nev.), on Wednesday. Because the Oval Office was cluttered with television cables, Reagan and Laxalt took their lunch in Deaver's little office nearby, where the president ate navy bean soup and orange sherbet and asked the senator, who had just returned from a western trip, what the folks out there thought about the Reagan administration.

Laxalt gave Reagan, whom he has known since they were neighboring state governors in 1967, an optimistic appraisal. He found the president enjoying his work and on top of the job, but said afterward, "It's amazing how insulated you get in those jobs and how quickly. He seemed to thirst for news from the outside world."

If there was a cloud on the White House horizon last week, it was a perennial problem which, in some past administrations, has become a presidential obsession: leaks of White House secrets.

Despite a dictum from Reagan and the senior staff that no one was to talk outside about the deliberations, the Cabinet meetings seemed to have the tightness of a sieve. Reagan aides and White House press secretary James S. Brady tried to locate the source of the leaks, which don't help an already overbusy Brady keep his contacts with the press.

And Brady was burdened with requests for articles, such as this one, of which Reedy wrote:" . . . This has led to the now venerable tradition in the press office of preparing a 'day in the life of the president' for handling the queries of reporters burdened with this story by somewhat unsophisticated editors."

Leaks aside, Reagan seemed to like what he was doing. When a White House aide told him that the response to his televised speech during the first hour was 378 positive and 24 negative responses, Reagan said, "What didn't the 24 like about it?"

He swapped jokes with visiting members of Congress, expressed his private enjoyment of House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.), and was moved by a gift from Nancy -- a framed picture of their daughter, Patti, embracing him when she left the White House. It was inscribed, "I love you, Dad."

For all this there is an air of unreality about the early Reagan White House, almost as if the Californians who are now in control can't believe what has happened to them. Meese and Deaver still forget sometimes and call the president "governor."

Early in the week political adviser Lyn Nofziger almost made the same mistake, and said it was difficult to remember that Reagan was not governor but president.

President replied: "We should set a day aside each week to call me governor, so we'll remember where we came from."

Reagan seemed to have the same problem. He told an aide that he was enthusiastic about the opportunity to put his ideas into action after all those years of speechmaking, but he also seemed the slightest bit surprised to find himself in the Oval Office.

When Deaver complimented Reagan after the Thursday television speech, the aide added: "Sometimes I have to pinch myself to see if this is real."

Reagan looked up and smiled.

"So do I," the president admitted.