The sleeping brain still evaluates sound
During sleep, the brain does not simply stop responding to the outside world. Sound signals can still reach areas that analyze them, but the brain’s response is generally less likely to become a conscious experience. This helps preserve sleep while leaving some capacity to react to a possible warning or other important event.
That evaluation is selective, not a deliberate decision. A familiar, continuous noise can become predictable, so it is less likely to trigger a strong response. A sudden change or a sound the brain recognizes as meaningful may prompt more processing and, sometimes, wakefulness.
Why the same sound affects people differently
Waking depends on more than volume. The sound’s pattern and personal meaning matter, as do the sleeper’s current sleep stage and how readily they wake. A loud, steady noise may blend into the environment, while a quieter but unexpected sound can stand out. A person’s name may be especially noticeable because it is familiar and personally relevant, though it does not wake everyone every time.
This is why “heavy sleeper” does not necessarily mean someone’s hearing has switched off. It describes a tendency to remain asleep through sounds that might rouse someone else. The brain is still receiving and sorting information; only some of it rises far enough to interrupt sleep.
