The latest beliefs are that the main purposes of sleep are to enable the body to rest, allowing time for repairs to take place and for tissue to be regenerated(再生). Lack of sleep, however, can compromise the immune system, cause depression and promote anxiety.
For many people, lack of sleep is rarely anything of choice. Some have problems getting to sleep, others with staying asleep until the morning. Despite popular belief that sleep is one long event, research shows that, in an average night, there are five stages of sleep. In the first light stage, the heart rate and blood pressure go down and the muscles relax. In the next two stages, sleep gets progressively deeper. In stage four, usually reached after an hour, the slumber is so deep that, if awoken, the sleeper would be confused and disorientated. It is in this state that sleep-walking can occur, lasting no more than 15 minutes. In the fifth stage, the rapid eye movement (REM) stage, the eyes move constantly beneath closed lids as if the sleeper is looking at something. During this stage, the body is almost paralysed(瘫痪的). This REM stage is also the time when we dream.
Sleeping patterns change with age. One theory for the age-related change is that it is due to hormonal changes. The temperature rise occurs at daybreak in the young, but at three or four in the morning in the elderly. Age aside, it is estimated that roughly one in three people suffer some kind of sleep disturbance. Causes can be anything from pregnancy, smoking, and stress to alcohol and heart disease.
Apart from self-help therapy such as regular exercise, there are psychological treatments, including relaxation training and therapy aimed at getting rid of pre-sleep worries and anxieties. Medication is regarded by many as a last option and often takes the form of sleeping pills.
What’s the best title of this passage?

A．Lack of Sleep	B．Change of Sleep
C．Patterns of Sleep	D．Benefits of Sleep

Which of the following can cause people’s lack of sleep according to this passage?

A．Their burden from illness and drinking.

B．Their incomplete sleep stages.

C．Their body temperature change with age.

D．Their damaged immune system.

Which of the following is true to the passage?

A．Nobody can escape lack of sleep.

B．A sleep-walker usually dreams before the sleepwalking.

C．In the REM stage sleepers can see things around but can’t move the body.

D．Regular exercise can better help improve sleep quality than sleeping pills do.