According to the author, many sleeping disorders are caused by ______.

A．Most people need less sleep when grown older.
B．Most people need seven and a half hours of sleep every night.
C．On average, people in the US today sleep less per night than they used to.
D．Most people need more sleep when they are under pressure.

A．sleep for at least eight hours per night
B．believe that we need less sleep as we age
C．adjust our activities to new environment
D．be able to predict the rhythms of our biological clocks