This research observed high school students in 1999 and 2008, in which start times were delayed for senior students in 2006 from 9 am to 10:30 am. The junior students that were observed did not have a change in start times. In 2008, seniors were less likely to report sleep loss on school nights and were less sleepy than the 1999 seniors or the 2008 juniors. It is important to note that over the eight-year time span, the amount of technology used in the bedroom rose over 15%, which increased sleep loss and daytime tiredness.
Source: Sleep and Biological Rhythms - Japanese Society of Sleep Research https://onlinelibrary.wiley.com/doi/epdf/10.1111/sbr.12003
Borlase, B.J., Gander, P. H., Gibson, R. H. (2013). Effects of school start times and technology use on teenagers’ sleep: 1999-2008. The Japanese Society of Sleep Research, 11(1), 46-54. doi: 10.1111/sbr.12003