单选题：根据以下材料，回答题Sound sleep occurs when one's internalclock is reg

根据以下材料，回答题
Sound sleep occurs when one's internalclock is regulated. Circadian rhythms (生理节奏) can becomecompromised quite easily. Many people with sleep troubles tend to varybedtimes. To avoid this common problem, sleep sufferers should go to bed andwake up at the same time every day. To help improve circadian rhythms,sufferers should enjoy 15 minutes of sun exposure the first thing in themorning.
A bedtime routine can help preventinsomnia(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 goodnight's sleep. Taking a warm, candlelit bath or drinking a cup of warm milk canalso help you prepare for deep sleep at night.
Stress can have a bad effect on the bodyand the ability to sleep. A daily exercise routine can help lower the body'sstress naturally. One can also manage stress by writing diaries. Writingdiaries can help people express their anxieties, plan for the future andimprove sleep. Other ways to lower stress include deep breathing exercises andprogressive muscle relaxation.
The light sending out fromcomputers, 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 theevening can help you fall asleep faster and stay asleep longer once bedtimearrives.
A perfect bedroom invites sweetsleep. Most people sleep better ina cool, dark, and quiet room. Lower the temperature in the room to 16 -20degrees for a peaceful night's sleep. A dark room can be accomplished byinstalling light blocking curtains or wearing a sleep mask. Noise can be reduced with eitherearplugs or a white noise machine. Finally, a comfortable mattress(床垫) and pillowsare essential to a good night's sleep.
What is the passage mainlyabout? A．How to get a sound sleep.
A．How to get a sound sleep.
B．How to lower body stress.
B．How to lower body stress.
C．How to regulate internal clocks.
C．How to regulate internal clocks.
D．How to manage anxieties.
D．How to manage anxieties.