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I'm unaware of any setting for this; not sure that it happens every time I turn the device back on but it is at least fairly common i.e. I notice that a screen looks overly bright and check. I normally keep the brightness level in the bottom half and it reverts to either the max or maybe near that.
iMac 21.5″, macOS 12.5
Hi desm1,
Thank you for participating in the Apple Support Communities. It sounds like your Mac has an ambient light sensor and is adjusting the brightness automatically. This article explains more about this and how to turn this off if needed: Change your Mac display’s brightness
"
Automatically adjust brightness
> System Preferences, then click Displays 
.If you don’t see the “Automatically adjust brightness” checkbox, you can manually adjust the brightness.
Manually adjust brightness
> System Preferences, then click Displays .Depending on the type of display connected to your Mac, you may also see a Contrast slider that you can use to adjust the display’s contrast.
"
Take care.
Hi desm1,
Thank you for participating in the Apple Support Communities. It sounds like your Mac has an ambient light sensor and is adjusting the brightness automatically. This article explains more about this and how to turn this off if needed: Change your Mac display’s brightness
"
Automatically adjust brightness
> System Preferences, then click Displays .If you don’t see the “Automatically adjust brightness” checkbox, you can manually adjust the brightness.
Manually adjust brightness
> System Preferences, then click Displays .Depending on the type of display connected to your Mac, you may also see a Contrast slider that you can use to adjust the display’s contrast.
"
Take care.
Screen brightness seems to re-set to very bright; is this common?
