In order to keep open the possibility of installing software and updates at night, the U.S. Air Force decided several years ago that it was a good idea to have all of its PCs programmed to come on during the middle of the night and stay on continuously for nearly 24 hours at a time. But Air Force officials have now decided that keeping 600,000 computers running around the clock may not be a good idea for the budget. The PCs finally will be getting some downtime, resulting in a yearly savings of about $17 million, or 9 cents a day per computer.