Early Monday morning, firefighters were called out to 512 Humboldt Avenue after alarms began to sound.  Nursing home staff found a light fixture in one of the bathrooms had caught fire at 2:20 a.m. — but a single fire sprinkler extinguished the fire.  There were no reported injuries.  According to firefighters, none of the residents were ever aware of the ordeal and were able to sleep through the night undisturbed. Firefighters say the incident is proof that they were well-protected by the sprinkler system, which was installed only a year and a half ago.