"From the onset of puberty until late teen years, the brain chemical melatonin, which is responsible for sleepiness, is secreted from approximately 11 p.m. until approximately 8 a.m., nine hours later. This secretion is based on human circadian rhythms and is rather fixed [by exposure to sunlight]. In other words, typical youth are not able to fall asleep much before 11 p.m. and their brains will remain in sleep mode until about 8 a.m., regardless of what time they go to bed."
http://cehd.umn.edu/CAREI/Reports/docs/SST-2002Bulletin.pdf First Longitudinal Study of Later High School Start Times
http://cehd.umn.edu/Pubs/Researchworks/sleep.html overview of research on school start times
http://cehd.umn.edu/CAREI/Reports/summary.html#SchoolStart original study papers
http://www.fcps.edu/fts/taskforce07/documents/index.htm a large collection of school start time studies and reports assembled by a task force for the Fairfax County, VA school board which is currently debating a later start schedule
A key concern in most schools considering later start times is the impact on after school sports and other activities. What's interesting is that the few studies so far actually show increased participation in after school activities, with better season win-loss records and fewer injuries in teams getting sufficient sleep.
A very recent study also found that a later school start time significantly reduced motor vehicle crashes, which are the top cause of death and serious injury to high school students:
"Average crash rates for teen drivers in the study county in the 2 years after the change in school start time [from 7:30am to 8:30am] dropped 16.5%, compared with the 2 years prior to the change, whereas teen crash rates for the rest of the state increased 7.8% over the same time period."
"One 2006 survey by the foundation revealed that 28 percent of high school students fall asleep at school and 51 percent have driven while drowsy."
"20 percent of high school students fall asleep in class on a typical day [with typical start times]"
"One of the anecdotal findings [after changing to a later start time] was that we noticed better attendance and less student sleeping in class that first hour"