(B)
Lights Out at Bedtime
There are plenty of good reasons to power up a personal desktop or laptop computer in the evening — writing e-mails，chatting on social networks or making purchases over the Internet，for instance．But various studies indicate that people who stare at a PC’s bright monitor shortly before going to bed probably sleep less soundly than they would otherwise because the light seriously affects their natural wake-sleep cycle．"They haven't proved it though，" says Dieter Kunz，the team's director at Charite's Psychiatric Clinic in St．Hedwig's Hospital in Berlin．
Similarly，exposure to just 10 minutes of normal bathroom lighting is enough to free the sleep hormone melatonin，which regulates people's natural wake-sleep cycle and makes them sleepy at nightfall．
Ten years ago，a team of British and American researchers detected a photo-pigment（光敏色素） in the human eye that signals to the body whether it is day or night，summer or winter．The photo-pigment is especially sensitive to blue light．
"The blue light more or less tells the body，'It's daytime，be awake，'" Kunz says．Monitors have a mostly cold white light content and scientists suspect the photo-pigment may react similarly to it．So the longer people look into the bright light，the more awake they become - and then sleep poorly．
A lot of people have difficulty "switching off" in the evening．About half of the women in Germany and a quarter of the men sleep poorly．
Environmental factors are only one of the possible causes．Job stress as well as personal and health concerns are also the main reasons for their sleep problems．
In addition，electronic objects' standby lights and indicator lamps can be annoying in the dark and affect sleep．"Sound scientific evidence is still lacking on this matter，" Kunz notes．
According to Kunz，those who cannot go without their computer in the evening should turn down the blue content if possible and lower the brightness somewhat．"But nobody does that because，after all，you want to concentrate while working at the computer and the light helps the brain．"
The passage is mainly about ____________．

A．natural wake-up cycles

B．the effect of light on sleep

C．personal and health concerns

D．functions of computer monitors

The underlined word “which” in Paragraph 2 refers to_____．

A．normal bathroom lighting

B．10-minute exposure to lighting

C．the release process of a chemical

D．a chemical affecting one’s sleep cycle

We can learn from the passage that ______．

A．the blue light has a calming effect．

B．the photo-pigment tells people day or night

C．women in Germany use computers more often than men

D．it has been proved that standby lights affect people’s sleep