Parents often complain that teenagers stay up too late and are almost impossible to wake in the morning.
For years, this behavior was blamed on laziness or phones, but scientists now believe that biology plays an important role.
Sleep is partly controlled by melatonin, a hormone that makes us feel sleepy when it gets dark.
Studies have shown that during the teenage years, melatonin is released later at night, so many teenagers simply don't feel tired until eleven or even later.
However, most teenagers still need eight to ten hours of sleep a night, which is hard to get when school starts at 7:30.
As a result, many students arrive at school without enough sleep and find it hard to concentrate.
In some schools, classes have been moved to a later time, and the results have been encouraging.
Students were found to sleep longer, miss fewer classes and feel less stressed.
Of course, later start times can cause problems for parents' work schedules and after-school sports, so not every school has been able to change.
Meanwhile, experts recommend that teenagers keep phones out of the bedroom and go to bed at the same time every night.