How do I permanently set the brightness setting on my iMac?

I've had my iMac for about a year and a half now. I have it in my bedroom and like to have the brightness setting turned all the way down, since I mostly use it at night. For some reason, every time I turn my computer on, it defaults back to the brightest setting and I have to manually turn it all the way down where I like it. Is this a glitch or is there something I need to properly set in a control panel somewhere?How do I permanently set the brightness setting on my iMac?
reset the PRAM by holding down Command + Option + P + R during machine boot until the computer automatically restarts again, then let go.

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