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Even when you're asleep, your ears continue detecting sounds around you. Your brain filters these sounds and decides which ones are important enough to wake you. That's why you can sleep through traffic but wake up when someone calls your name or a baby cries.
This protective system helps you remain aware of potential danger while resting. Your hearing doesn't turn off—it simply becomes more selective. Your brain keeps listening even while you're dreaming.
© National Institute on Deafness and Other Communication Disorders