Why does the day-night sound level add a penalty to nighttime noise?

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The day-night sound level (DNL or $L_{d n}$ ) incorporates a 10-dBA penalty for noise occurring between 10 pm and 7 am because people are generally more sensitive to noise during nighttime hours. This adjustment accounts for the increased impact of noise on individuals during periods typically reserved for sleep and rest.1

By adding this penalty to sound pressure level (SPL) values measured during the night, the DNL provides a more accurate representation of overall community exposure to noise, such as that generated by aircraft.1