adolescents. Several school districts in the US have adopted later start times for their high schools because of this.

Some related links:

http://www.medicalnewstoday.com/articles/133246.php

http://www.stanford.edu/~dement/adolescent.html

http://cehd.umn.edu/Pubs/Researchworks/sleep.html

"Most plain girls are virtuous because of the scarcity of opportunity to be otherwise."
-- Maya Angelou