A simple schedule shift—letting teens start school later—could boost sleep, health, and grades. Credit: Shutterstock Many high school students find it hard to fall asleep early, which makes waking up for early morning classes a challenge. The reason lies in biology. During adolescence, the body's internal clock naturally shifts later, meaning teenagers tend to feel alert later at night than adults do. Because of this shift, many teens cannot fall asleep early enough to get sufficient rest