I'd forgotten, but there is an alternative method.
https://www.mobileread.com/forums/sho...d.php?t=257246
I have not used it, but instead of using the brightness control, this puts icons on your Kobo homescreen which would toggle nightmode on and off. It should work for the Glo HD since it was developed for Kobos which lacked the physical button to turn the light off and on.