【单选题】For millions of years before the appearance of the electric light, shift work, all night cable TV and the Internet, Earth's creatures evolved on a planet with predictable and reassuring 24-hour rhythm...
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.