Researchers indicate that due to hormonal changes during puberty, teenagers feel a biological need to go to bed and wake up later than others. The circadian rhythms tend to shift and the hormonal changes delay secretion of melatonin, a hormone that promotes sleep. This sleep phase delay causes people to feel sleepier as the day progresses. This is the reason some of U.S. public high schools have been considering moving the school start times later to help them cope with mental health and behavioral challenges. A recent study published in the journal Sleep suggests that students with a later school start get enough sleep and therefore do well in their studies. According to the Centers for Disease Control (CDC), the number of U.S. high school students who sleep less than eight hours grew to 78% share of public schools in the U.S. 

Additional reference: https://sleepresearchsociety.org/publications/journal-sleep/