【Whycansomepeoplesleepthroughnoiseslikeahonkingcarorflushingtoilet，whileothersareawakenedbythelightestsound？Tofindtheanswer，sleepresearchersatMassachusettsGeneralHospitalconductedanunusualstudyof12self-described】
Why can some people sleep through noises like a honking car or flushing toilet，while others are awakened by the lightest sound？
To find the answer，sleep researchers at Massachusetts General Hospital conducted an unusual study of 12self-described deep sleepers．After tests confirmed that the healthy volunteers were solid sleepers，they took part in a three-night study in the university's sleep laboratory．The participants spent the night in a big and comfortable room．But the room also included four speakers positioned near the top of the bed．
During the night，the deep sleepers were subjected to 14different recorded sounds，like street traffic，toilets flushing，an ice machine dispensing and an airplane flying overhead．Next door，the researchers monitored their sleep patterns and brain waves．
As expected，all of the participants slept relatively well，but there were differences in how they responded to the noisy interruptions．Some of the sleepers didn't wake up even when a sound was blasted at 70decibels （分贝）； others were awakened by sounds at 40or 50decibels．
The researchers discovered that the difference in a sleeper's reaction to noise could be predicted by the level of brain activity called"sleep spindles''．A sleep spindle is a burst of high-frequency brain activity coming from deep inside the brain during sleep．The source of the spindles is the thalamus，a part of the brain that sends sensory information to the rest of the cortex （皮层）．
Before the study，the Massachusetts researchers theorized （推理） that the spindles are the brain's way of preventing sensory information from passing through the thalamus and waking the rest of the brain during sleep．They found that the sleepers who experienced the most sleep spindles during the night were also the soundest sleepers and were least likely to be awakened by noise．
Scientists already know that most people become lighter sleepers with age，most likely because older peop