Maria Popova, whose invaluable site Brain Pickings compiles fascinating and diverse material on books, the arts, and sciences, has written and excellent review of a book entitled Internal Time: Chronotypes, Social Jet Lag, and Why You’re So Tired by Till Roenneberg.
The book explains the clash between circadian (body clock) biology and school/work schedules, resulting in a persistent state of sleep deprivation the author calls “social jet lag.” She summarizes the findings of the book more elegantly than I can. You should go read this. She describes the physiologic shift in teenager’s schedules to a later sleep schedule and how that clashes with earlier school start times. This results in many sleepy teenagers and significant health and school problems.