Canton High School students may have some extra time in the morning to sleep next fall, while those at Galvin Middle School may have to rise earlier. The Start Time Committee, made up of administrators, School Committee members, teachers, and parents, has been looking for the past 18 months into whether the high school should begin later, according to Superintendent Jeffrey Granatino. Students reaching adolescence go to bed later, partly because their bodies and sleep patterns are changing, Granatino said in an interview Wednesday. Currently, the high school begins at 7:20 a.m. and the middle school begins at 7:45 a.m. The most likely recommendation of the committee will be to reverse those times, but that decision has not yet been made, he said. The committee will make its recommendation before the School Committee in the next month.