A teen's internal clock
"Everyone has an internal clock that influences body temperature, sleep cycles, appetite and hormonal changes. The biological and psychological processes that follow the cycle of this 24-hour internal clock are called circadian rhythms. Puberty changes a teen's internal clock, delaying the time he or she starts feeling sleepy and awakens.
Too little sleep
Most teens need about nine hours of sleep a night — and sometimes more — to maintain optimal daytime alertness. But few teens actually get that much sleep regularly.... READ MORE FROM "THE MAYO CLINIC"