DARLINGTON - Pulling an "all-nighter" - staying up all night studying for exams or doing school work - is considered a rite of passage for many college students. But new research shows that relying on all-nighters too often can adversely affect students' grades and overall health.Getting less than six hours of sleep per night can lead to deficits in attention, concentration, memory and critical thinking, along with increased depression, irritability and anxiety. "A student might feel sick, fatigued, anxious or unable to concentrate, but they will rarely complain about lack of sleep as the reason," Mary Knellwolf, University of Wisconsin-Extension family living educator in Lafayette County, said.