Elderly people may fight to sleep as well as they did during their youth because they start to lose part of the brain that helps them fall asleep at night, scientists believe. There seem to be two main difficulties for the elderly： falling asleep at the start of the night easily and then waking up too early in the morning - yet finding it almost impossible to get back to sleep.
  Researchers have found out a group of cells （细胞） in the human brain which can help us sleep. They think these neurons （神经元） are slowly destroyed as we age, leading to greater sleep problems.
  Clifford Saper, a professor who led the research, said, "A person in his 70s has about one hour less sleep per night than a person in his 20s. The loss of these neurons with aging may be an important reason why older people often face sleep problems."
  Professor Saper's team discovered a group of neurons in rats - the ventralateralpreoptic nucleus （VLPO）， which works as a sleep switch in rats. They then found the same process was happening in humans. The cells turned off the brain's arousal （唤起） systems to enable the animals to fall asleep.
  Professor Saper said, "Our experiments in animals showed loss of these neurons produced severe sleeplessness, with animals sleeping only about 50 percent as much as normal and their remaining sleep being interrupted （打扰）."
  The researchers analyzed data from 45 elderly people and found people with fewer neurons in the "sleep switch" area were more likely to have sleepless nights.
"The fewer the neurons are, the more broken the sleep becomes. These findings provide the first evidence that VLPO in humans probably plays a key role in causing sleep. These results may, therefore, lead to new methods of dealing with sleep problems of the elderly." Professor Saper said.



According to the passage, one of the elderly people's sleeping problems is that ______ .

A. they can't fall asleep till late into the night
B. they can't fall asleep at the start of the night
C. they often wake up too early in the morning
D. they often have terrible dreams while asleep

What can VLPO do according to the passage？ ______

A. Help people fall asleep.
B. Stop people from sleeping.
C. Control people's feelings.
D. Enable people to become happy.

What does the underlined word "severe" mean？ ______

A. Natural.
B. Serious.
C. Separate.
D. Safe.

What is the passage mainly about？ ______

A. The findings of a piece of research.
B. The main difficulties of the elderly.
C. Ways to deal with sleeping trouble.
D. Ways to help the elderly improve their sleeping quality.