Passage One
Sound sleep occurs when one's internal clock is regulated. Circadian rhythms（生理节奏）can become compromised quite easily. Many people with sleep troubles tend to vary bedtimes. To avoid this common problem, sleep sufferers should go to bed and wake up at the same time every day. to help improve circadian rhythms, sufferers should enjoy 15 minutes of sun exposure the first thing in the morning.
A bedtime routine can help prevent insomnia（失眠）. One hour before bedtime, one should lower the lights and relax. Reading a book or listening to calm music can help prepare the body for a good night's sleep. Taking a warm, candlelit bath or drinking a cup of warm milk can also help you prepare for deep sleep at night.
Stress can have a bad effect on the body and the ability to sleep. A daily exercise routine can help lower the body's stress naturally. One can also manage stress by writing diaries. Writing diaries can help people express their anxieties, plan for the future and improve sleep. Other ways to lower stress include deep breathing exercises and progressive muscle relaxation.
The light sending out from computers, cellphones, tablets and televisions can cause sleep problems. Therefore, these items should be turned off at least an hour before bedtime. Additionally lowering the backlight on these electronic devices earlier in the evening can help you fall asleep faster and stay asleep longer once bedtime arrives.
A perfect bedroom invites sweet sleep. Most people sleep better in a cool, dark, and quiet room. Lower the temperature in the room to 16~20 degrees for a peaceful night's sleep. A dark room can be accomplished by installing light blocking curtains or wearing a sleep mask. Noise can be reduced with either earplugs or a white noise machine. Finally, a comfortable mattress（床垫）and pillows are essential to a good night's sleep.
According to Paragraph 2, which of the following may lead to insomnia?