Restaurants that once served two distinct meals a day, lunch and dinner, are acting more like diners, opening early in the morning and keeping their kitchens busy late into the night, and serving in the traditionally slow times between meals. And places that used to close one or two days a week to give the staff a night off now see that as a luxury they can no longer afford. The shift toward all-hours dining has been going on for some time. In part, it reflects the busy lives of New Yorkers, who may start the day with a business meeting over scones and lattes, or spend the afternoon answering e-mail in one of the restaurants around town that offer free wireless connections. But the trend has been picking up speed this fall, as restaurateurs try to cope with the lower checks racked up by diners who have become noticeably more frugal.