The doors of the Mission Neighborhood Health Center in San Francisco don't open until 7 am, but on the Saturday morning I was there, a dozen or so people were already lined up by 5:30. The group included a middle-aged white man who had lost his job managing a high-end restaurant and a black man wearing a crisp security guard blazer because he had to be at work by noon. Each was there hoping for a bed for the night. The city assigns most slots in its homeless shelters on a first-come, first-served basis by computer. The people had shown up here so early because they know through experience that every last bed will be claimed by 7:10 am.