Redshift shifts the color palette of the screen towards the red spectrum at nighttime (and undoes it during daylight). To dim as well the (external) screens on Linux hooks are called for; citing man redshift:

Executables (e.g. scripts) placed in folder ~/.config/redshift/hooks will be run when a certain event happens. The first parameter to the script indicates the event and further parameters may indicate more details about the event. The event period-changed is indicated when the period changes (night, daytime, transition). The second parameter is the old period and the third is the new period. The event is also signaled when Redshift starts up with the old period set to none. A simple script to handle these events can be written like this:

#!/bin/sh
case $1 in
	    period-changed)
	        exec notify-send "Redshift" "Period changed to $3"
esac

Solution

The following script (named, say, brightness.sh, marked executable by chmod a+x brightness.sh and put into ~/.config/redshift/hooks) sets the brightness levels $brightness_day/transition/night on the respective period changes of

• the laptop screen via xbacklight and
• every external screen via ddcutil

#!/bin/sh

# Set brightness values from 1 to 100 for each status
brightness_day="90"
brightness_transition="55"
brightness_night="35"

# In case computer wakes up and older brightness.sh process running
killall --user "$USER" --older-than 1h --exact 'brightness.sh'

# Adjust brightness of internal screen via xbacklight
if command -v brightnessctl >/dev/null 2>&1; then
  set_brightness_internal() { brightnessctl set "$1"%; }
elif command -v brightnessctl; then
  # Set fade time to one minute
  fade_time=60000
  steps_per_second=15
  steps=$(("$fade_time" * "$steps_per_second" / 1000))
  set_brightness_internal() { xbacklight -set "$1" -time $fade_time -step "$steps"; }
else
  set_brightness_internal() { return; }
fi

# Adjust brightness of external screen via ddcutil
if command -v ddcutil >/dev/null 2>&1; then
  set_brightness_external() {
    ddcutil setvcp 0x10 $1
  }
else
  set_brightness_external() { return; }
fi

case $1 in
  period-changed)
    case $3 in
      night)
        set_brightness_internal $brightness_night
        set_brightness_external $brightness_night
        ;;
      transition)
        set_brightness_internal $brightness_transition
        set_brightness_external $brightness_transition
        ;;
      daytime)
        set_brightness_internal $brightness_day
        set_brightness_external $brightness_day
        ;;
    esac
    ;;
esac

Caveat

As of now redshift’s hooks do not run on many distributions due to a faulty set-up of AppArmor inhibiting these. There are three options:

• use GammaStep, forked from Redshift, to overcome such shortcomings, or
• wait for this fix to be included in Redshift and tickle down on Debian and other major distributions,

• fix it yourself by adding

    owner @{HOME}/.config/redshift.conf r,
    /etc/passwd* r,
  

  to /etc/apparmor.d/local/usr.bin.redshift by sudoedit /etc/apparmor.d/local/usr.bin.redshift and running sudo systemctl reload apparmor.service.