By nightfall Saturday, Brian Luschwitz will want to go sleep.

Really, really want to go to sleep.

There will be the reason everyone can relate to -- darkness encourages drowsiness.

And there will be the reason only true running fanatics can relate to -- Luschwitz will have been on his feet, running, since 5 a.m.

Luschwitz, a 41-year-old pediatrician, will run the Angeles Crest 100-Mile Endurance Run on Saturday and Sunday to raise money for the Vista Community Clinic, where he's worked for six years.

Over two days, he will run nearly as far over a rugged mountain range as he drove from his house in Escondido to the start of the race in Wrightwood.

He will wind through 100 miles of trails and up to 10,000 feet in elevation before finishing in Pasadena. He will rely on a flashlight, trail maps and instincts to stay on the correct trail when darkness covers the landscape.

And he will not sleep, he said, until he crosses the finish line.

This will be his fourth time starting the grueling race. Two times, he ran straight through his grogginess and finished.

But two years ago, he lay down for a "short nap," which turned into the three letters every runner dreads most next to his name: "DNF." Did Not Finish.

"I enjoy running, but it gets kind of grueling," he said. "You just have to be persistent and get to that finish line."

He stays motivated, he said, thinking about all the people who have sponsored his runs with donations for his clinic. And he said he thinks about setting a good example for his two young sons, Luke and Sean.

This year, he's hoping to raise $10,000 to help support a medical clinic being built on Grapevine Road by the Vista Community Clinic.

The new $6 million, 12,000-square-foot clinic will house six doctors and nurse practitioners. It will double the size of the current leased building, he said, and will serve up to 8,000 more patients a year.

In the current space, there's just one waiting room for all patients.

"They're practically sitting on top of each other," he joked.

In the new clinic, there will be separate waiting rooms for adults, kids and sick kids, he said.

Luschwitz sees 5,000 patients a year, most who have no insurance or inadequate coverage. But they treat everyone. And he prefers the free-clinic approach.

"As a doctor, I find it very fulfilling," he said. "My patients are very grateful for the care we give them."

However, he said funding is always short, and he does the runs as "attention-grabbers."

He started running somewhat accidentally about six years ago, he said. His brother-in-law, who lived next door, said "Hey, let's do a marathon."

Five months later, they did, slogging through a 26.2-miler in Carlsbad in five hours.

By the following November, he was severely hooked and made his first 100-mile jaunt near Lake Moreno in East County.

It got below freezing overnight and winds whipped to 55 mph.

Luschwitz's response: "Hey, this is a great challenge!" He finished in fourth place -- only 11 finished -- and joked, "I'm great at this!"

The long races continued, and he got an idea to marry his obsession with long runs to fund-raising for his clinic from a team of marathoners that raises money for leukemia research.

This year, he cut back his training a few weeks short of the race to visit China, his wife's native country, and watch the Olympics.

Watching a team of Chinese kids taking turns walking a wheelchair-bound friend up the Great Wall inspired him, he said.

"A group of enthusiastic people were helping someone less fortunate get to a place she could never have gone on her own," he said.

It reminded him of his clinic, he said, and will be his motivation to keep going when dusk comes Saturday and there are still miles to go before he sleeps.

Contact staff writer Dan Simmons at (760) 740-5426 or dsimmons@nctimes.com.