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The issue:
The screen appears about 2-3 dots dimmer on max brightness after waking from sleep for more than 30 minutes. I only noticed this a few months after upgrading to Catalina, it's been ongoing ever since.

My settings:
No auto-adjusted brightness, disabled screen temperature modifiers like NightShift/Flux, slightly higher gamma color profile for better contrast/brightness (it looks a bit faded otherwise).

My computer:
MacBook Pro 13" early 2015 mid-tier model with Catalina 10.15.4. (bought in 2017 used, no repairs or hardware issues the whole year. Great battery cycle life).

My Temporary solution:
Resetting the PRAM (Cmd+Opt+P+R). This fix lasts until I close my laptop and let it sleep overnight usually. Sometimes I've had success reducing the brightness before putting it to sleep. I am not completely sure if this is hardware or software-related.

There have been rare/infrequent occasions when my display would do weird things after waking from sleep. There was one incident of my display completely blanking out with a white pixelated screen for a moment (like an old static TV out of tune). A few other incidents where the display blinks black or totally freezes. It was at first a problem with video playback and hardware acceleration, but now I am beginning to believe it may be a hardware-related problem (though this Mac doesn't have a graphics card).

2 Answers 2

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Just figured this out for my case. I had already disabled the brightness from automatically adjusting. Every time my computer woke up from sleep however the brightness would still drop to around 50%.

Turns out I also needed uncheck the box to "Slightly dim the display while on battery power"

System Preferences > Battery > Slightly dim the display while on battery power

See the image below:

uncheck auto dim on battery

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  • Good find on these close questions - thanks for the clear answer.
    – bmike
    Commented Feb 18, 2021 at 22:38
  • Already disabled for me when not plugged in and still dimming. Also turned off the Automatic setting in display.
    – Chucky
    Commented Jul 15, 2022 at 15:02
  • Still not fixed for me. After closing and opening the lid, the brightness goes down. All automatic settings are off. Commented Jun 28, 2023 at 12:40
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The above "Slightly dim the display while on battery power" was already disabled on my MBP, and still had the same issues.

The problem for me was in Battery > Low Power Mode was set to "Always".

Switched to "Only on battery" (or "Never") and no more dimming on wake up.

switching low power mode settings from always to only on battery

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