While most Harvard students cagerly look forward to December vacation, student organizers of one Cambridge homeless shelter are once again bracing for a lack of volunteers during the holiday season.

Every year, the student-run University Lutheran Shelter has problems finding enough volunteers to staff its 23-bed facility when students leave Cambridge.

Finding volunteers during the winter holiday is "the most difficult because it's a long vacation and everyone leaves," said Michelle S. Sandberg '95, the shelter's director of volunteering.

Volunteers from the University Lutheran congregation, Harvard employees and members of the Cambridge community will help make up for the loss of student participants, Sandberg said.

In addition, a core group of four or five student volunteers, most of whom are from the Boston area, will volunteer regularly during the holiday season.

But Sandberg said that will not be enough to replace the students who will leave during winter break. During the semester, 75 students volunteer at the shelter.

"More volunteers are still needed," Sandberg said. "The holidays are a very difficult time for homeless people and they really need the stability of a shelter."

The shelter, a Phillips Brooks House Association committee, is run, organized and staffed almost exclusively by Harvard undergraduates.

Officials at numerous Cambridge homeless shelters contacted yesterday said they experience a slight increase in residents during the winter.

But the University Lutheran Shelter will not experience an increase because it can never handle more than 23 residents, Sandberg said. The shelter turns away people all year, she added.

Homeless people interviewed yesterday said they are bracing for winter. One homeless man described his morning battle with the cold.

"I woke up with my arm frozen to the ground," said the man, who requested anonymity.