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Think about it this way. When you are in darkness(like at night) do you use hearing to help you know where you are going? People who have lost their sight-they will not see with their eyes, they will see what is around them with using touch or hearing. Their hearing will compensate for what they cannot see.
Because owls are generally active at night, they have a highly developed auditory (hearing) system
The length of time a battery lasts depends quite a bit on how strong the hearing aid is....how high the volume is set. This is the #1 determinant of battery life. BUT, yes, your mother should open the battery door at night. This is standard, good operating protocol. Not only is the hearing aid definitely shut off, but it also allows air to get into the inner parts of the hearing instrument and dry it out so it's dry and ready to go in the morning. Many of the new hearing aids do not have the traditional on/off switches anymore.....so the aid might well still be on as long as the battery door is closed. Not knowing what make or model hearing aid she has, this is a strong possibility
Silent Hill.
Piano
our naked eyes...
periscope
Radar
Before people had invented artificial light it was so dark at night that well... If you couldn't see and you kept hearing noises in the night would you think it was a flesh-eating monster or a cute little bunny? -Artixthepaladin
Usually during the night.
You may want to check your alternator to make sure it is producing enough charge to run everything at night.
Maybe you are hearing your cat purring. This is what cats do when they are comfortable.