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 12 self-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 14 different 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  ly 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 70 decibels  ; others were awakened by sounds at 40 or 50 decibels. 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  . Before the study, the Massachusetts researchers theorized   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 people experience less "slow wave sleep'', which is the deepest stage of sleep. People also produce fewer sleep spindles as they age. But even when controlling for the stage of sleep a person was in, the number of sleep spindles still predicted their risk for awakening because of noise. More research is needed, but the findings suggest that a better understanding of sleep spindles could lead to new behavioral or drug therapies for people with sleep disorders. For instance, future studies may try to determine whether diet, exercise or other behaviors may influence the number of sleep spindles a person produces during the night. Some participants can sleep well through loud noises mainly because   _  .
their brains don't respond to outside noises their brains react differently to noises they adapt to the environment quickly they don't pay attention to the monitors