When the body is awake and the senses are functioning properly, a person constantly encounters new stimuli to sense and so the impressions of previously perceived stimuli are ignored.
wˌɛn ðə bˈɑːdi ɪz ɐwˈeɪk ænd ðə sˈɛnsᵻz ɑːɹ fˈʌŋkʃənɪŋ pɹˈɑːpɚli, ɐ pˈɜːsən kˈɑːnstəntli ɛŋkˈaʊntɚz nˈuː stˈɪmjʊlˌaɪ tə sˈɛns ænd sˌoʊ ðɪ ɪmpɹˈɛʃənz ʌv pɹˈiːviəsli pɚsˈiːvd stˈɪmjʊlˌaɪ ɑːɹ ɪɡnˈoːɹd.