If you want to do this in editor with action parts then you need to use a combination of pause action parts & a jump to x action part to create a loop. You need to remember to quit the action you looped or also add an if query in there with a condition or value to kill the loop automatically.
Ok, so... this is more or less what the actions would look like...
set brightness '100'
pause 1000 ms
set brightness '40'
pause 1000 ms
jump to action part #1
If you wanted to go down the scripting route then you could actually fade in / out to the new value over x time with easing by using the to() tweening function.
example... (inside an execute a script action part)
game.CurrentScene:to(1000, {Brightness = 100}, easeQuintInOut)
P.S: if you want to get really brave then you could use the set random value action part to generate random delay / pause times &/or for setting a random brightness value.