You can do this using
with a script such as:
intensity:=50 ; just a number
Gui, 1: Default
Gui, Color, 0x000000 ; Color to black
Gui, +LastFound +AlwaysOnTop -Caption +E0x20 ; Click through GUI always on top.
WinSet, Transparent, %Intensity% ; Set intensity first based on the time of dday
Gui, Show, x0 y0 w%A_ScreenWidth% h%A_ScreenHeight% ; Cover entire screen
This will create click-through window on the primary screen, located
in the x,y coordinates of 0,0 and covers the entire screen with a transparency factor
By changing the displayed coordinates you may choose another monitor,
which is just part of the extended desktop with its coordinates of X, Y,
width and height.
To discover the coordinates of your monitor, use the
You will need to study AutoHotkey, but you may find a lot of information available
on the Internet.
Once you know something about AutoHotkey, you may tackle
the AutoHotkey script
This script lets you control brightness using the mousewheel control, by
holding down the left button mouse while scrolling up or down.
As above, it adds a black overlay over the screen of varying transparency.