While Washington slept yesterday morning, the man they called President Reagan stood erect in the bone-rattling predawn cold and rather sheepishly took the oath of office.

At his side on the Capitol platform stood the man they called Nancy Reagan, a six-footer in uniform who later speculated that his weight (150 pounds) would mortify the first lady were it indicative of her own.

When it comes to the dress rehearsal for the Jan. 21 swearing-in ceremonies and inaugural parade, giant stretches of the imagination are required. Thus, a 21-year-old Army private first class named Thomas Browne becomes 73-year-old Ronald Reagan; a 20-year-old Army private, William Koons, stands in for the elegantly clad Mrs. Reagan, and a fleet of limousines and trucks represents the husky dog sled team, the drum and bugle corps and the high school bands that will make their way down Pennsylvania Avenue next Monday.

"The purpose is to check the logistics, the timing, the cadence, the pace," said Maj. P.J. Crowley, a military spokesman. "A lot of these people have never participated in anything like this before. There are cues they have to listen to like an actor."

An indifferent half-moon looked down from a blue-black sky and the temperature was in the merciless mid-20s as the players gathered at the Capitol at 5 a.m. to rehearse the big day.

The early hour, the occasional flood of television lights on the platform and the soft crunch of military boots in the frozen snow gave the scene a surreal quality, intensified by Army trumpeters on a balcony above and the stately strains of "Hail to the Chief" from the Marine Band below.

But if the atmosphere at that hour seemed otherworldly, the intent behind the gathering was practical. Timing may well be the most crucial consideration on Jan. 21, and yesterday morning some 200 people equipped with clipboards and walkie-talkies worked to ensure that punctuality reigns on Inauguration Day.

"Timing is so important because the president is on a very tight schedule," said Crowley. "A Fourth of July parade falls behind and the only thing to worry about is the crowds on the curbside. Here, there are other ramifications -- one thing falls behind and so does everything else."

The swearing-in ceremony is set for noon, followed by the inaugural address. President Reagan and other dignitaries will then retreat for lunch, after which they will make their own miniparade along the route to the reviewing stand at the White House. At 2 p.m., the big parade begins.

Four years ago, the parade was budgeted for one hour; the final unit passed by the White House reviewing stand at 59:54, six seconds early, Crowley said. Next week's inaugural parade with its estimated 150 units is budgeted for 1 hour 15 minutes.

While parade coordinators tried their best in yesterday's maneuvers to simulate the swearing-in ceremonies and the parade, some things, of necessity, were modified.

The 21-gun salute at 6 a.m. was reduced to a first and last shot because, said USMC Sgt. Sylvia Gethicker, "We might get a few neighbors' complaints otherwise."

The celebrated 700-plus horses that are scheduled to participate in the real parade were presumably home yesterday in warm stables.

And President and Mrs. Reagan and the other dignitaries who have a place on the swearing-in platform were not present. The 12 stand-ins were selected from the Old Guard, the Army's ceremonial unit from Fort Myer, with one exception: Gerald Vaughan, 36, a District resident who is blind, said he requested a chance to sit on the freezing platform during the rehearsal because of his interest in history. Vaughan, who is affiliated with the Lighthouse for the Blind, stood in for one of four ministers.

For their part, the military stand-ins diplomatically avoided sinking too deeply into their let's-pretend game.

Although Pfc. Browne was heard to summon his mate to his side with a peremptory, "Nancy!" he merely smiled when asked what he would do if he were really president, and he quickly backed away when the questions persisted.

"It's a new experience to be president," he said as he made his escape.