In the darkest hours of the night, Judith Kaplan, dressed in her Sabbath finery, sat in a tent outside the New York City Medical Examiner's office, singing the haunting repertoire from the Book of Psalms. From midnight until 5 a.m., within sight of trucks full of body parts from the World Trade Center, she fulfilled the most selfless of Jewish commandments: to keep watch over the dead, who must not be left alone from the moment of passing until burial.

Normally, this Orthodox ritual, known as sitting shmira, lasts for only 24 hours and is performed by one Jew, customarily a man, for another Jew. But these are not normal times. Thus the round-the-clock vigil outside the morgue on First Avenue and 30th Street is already in its eighth week. The three sealed trucks may or may not contain Jewish bodies. And the shomer, or watcher, is just as often a young woman as an old man.

Ms. Kaplan, 20, a senior at Stern College for Women, a division of Yeshiva University, is one of nine students who have volunteered for this solemn task on weekends, working in shifts from Friday afternoons until nightfall on Saturdays, the holiest part of the week. The rest of the time, the task is performed by scores of volunteers from an Orthodox synagogue, Ohab Zedek, on West 95th Street.

Devout Jews cannot ride on the Sabbath, putting the subway or taxis off-limits for the long trek from Ohab Zedek to the morgue. So the Stern students, whose dormitories are within blocks of the morgue, have filled the breach. They were recruited by Jessica Russak, 20, a student who takes the dawn shift, peeking out of the tent as the sky brightens to time her morning prayers.

Ms. Russak, Ms. Kaplan and the others have won blessings from Christian chaplains at the site, and their dedication has moved police officers and medical examiners to tears. The burly state trooper who guards the area has learned the girls' names, and a bit about their religion.

At first, the trooper demanded identification, not knowing that carrying anything on the Sabbath was prohibited for Orthodox Jews. Now he keeps an eye on the prayer books and snacks that the Stern students drop off before sundown on Friday and retrieve Saturday night. The trooper once called Ms. Russak at home when she was a few minutes late, in case her alarm clock had not gone off.

The young women have the full support of Dr. Norman Lamm, president of Yeshiva University, who agreed without hesitation that the normal gender rules — women can sit shmira only for other women, while men can sit for any deceased person — could be waived under the circumstances. The school is also providing security guards to escort those who sit the late-night shifts.

While the tradition is a peculiarly Jewish one, Dr. Lamm said he felt that the mitzvah, or good deed, reached across denominations. "The idea that you can have companionship even in death is a very consoling thought, whether you are Jewish or not," he said. Dr. Lamm called "the loving watching of the corpse a very human act" and noted that the shmira is "the truest and most sublime" of the 613 mitzvahs "because there can never be reciprocity."

But there are other rewards, which the Stern students discussed on Friday, at Ms. Kaplan's apartment, while preparing their Sabbath dinner — four different kinds of kugel, pepper steak and honey-glazed chicken.

All of them had felt so helpless after the terrorist attacks. They donated money to the Red Cross, but were turned away as blood donors or volunteers because those needs had quickly been met. Then came the pleas for Sabbath shomers. "This is something I can do," Ms. Kaplan said.

"And it's surreal. You absolutely feel the souls there, and you feel them feeling better."

Each volunteer said she had begun with fears about sitting within sight of the trucks full of remains. Instead, they said, they have found peace and a kind of joy.

Ms. Russak does not sing the psalms as Ms. Kaplan does, but rather mutters them, in whatever order moves her, often starting with No. 130, which she knows by heart. The effect is meditative. "The meter and the rhythm, one after the next after the next, it calms you," Ms. Russak said. "That's the magic of the psalms. They put you in the right place."

Ms. Kaplan made up slow, sad tunes for each psalm and sings them in a clear soprano, sweet as birdsong. If she mumbled them, without melody, Ms. Kaplan said, she might lose a word here and there and thus the full meaning of each line. By singing, she said, she is fully mindful.

"Time completely stops," she said. "Now I understand what it is to pray with your heart."

Two weeks ago, during her regular four-hour shift, Ms. Kaplan sang 128 of the 150 psalms and grudgingly gave up her place to Ms. Russak at 4 a.m., begging her to finish the cycle. Last week, determined to do the full canon on her own, Ms. Kaplan pleaded and won an extra hour.

"It's very completing for her," Ms. Russak said. "Like finishing an entire book of the Torah."

But before Ms. Kaplan's middle-of- the-night vigil on the brown leather benches in the tent, others had taken their turns, among them Anat Barber, the newest recruit, who was full of nervous questions. "The bodies there, do they know who they are?" Ms. Barber asked, as Ms. Russak escorted her to the site for the first time.

Ms. Russak did her best to be reassuring, telling Ms. Barber that she would be fine, that "the irony is that it feels too easy." Outside the tent, the last of the men, a volunteer from Ohab Zedek, was rushing toward his Sabbath observance in Brooklyn. It was time for the women to begin their watch, to fill the night with poetry and prayer.