It is pointed out in the passage that a sleep time under 8 hours ------ .

The effects of sleep loss are subject to a number of
popular misconceptions. The belief that everyone
must sleep 8 hours a night is a myth. According to the
results of a recent survey on the subject, adults
average about 7 to 7 1 / 2 hours of sleep per night, and
many individuals function effectively with 5 to 6 hours
of sleep. In fact, 20 percent of the population (slightly
more in men) sleep less than 6 hours per night.
Another significant fact is that sleep time decreases
with age.