Moral of the story: Ever wonder how your roommate can sleep through fire alarms, car horns, and other crazy loud things?  Surprise surprise (not really) you brain hears everything but some block those stimuli.

Source: AMA Morning Rounds 8/10/2010

Some brains may be specifically wired to block out noise during the night.

USA Today (8/10, Steinberg) asks, "Why can some people sleep through car alarms and thunderstorms when others wake up at the sound of footsteps?" Harvard researchers now "believe they've found the answer tucked in sleep spindles, or bursts of brain activity that occur only during sleep."

        Before reaching that conclusion, investigators conducted a "three-night study," inviting "12 volunteers who reported being deep and healthy sleepers into a sleep lab with a comfy queen-size bed outfitted with enormous speakers at the headboard," Time(8/10, Park) reports. The "researchers recorded the participants' brain waves as they slept normally the first night, and then on subsequent nights as they were bombarded with 14 different noises...which were played at progressively louder volumes." The team "paid particular attention to the patterns generated by the thalamus, a region deep in the brain that processes incoming visual and auditory stimuli."

        The team had surmised that "the thalamus might generate" the "sleep spindles as a way to prevent sensory input (such as loud noise) from reaching the sleeping brain," HealthDay (8/9, Phillips) reported. According to the paper in Current Biology, "people with higher rates of spindle rhythms were consistently less likely to awake in response to these noises." But, it's still not "completely clear if sleep spindles are directly interfering with sound transmission to the brain, although that's the current hypothesis."