Just realized how good it would be to have an Android-like 'Bedtime Mode" on my desktop to tell me to "Go to bed!" by making my computer go completely grayscale.

Sad to see that KDE doesn't have the option, and Gnome requires an extension for the functionality.  

KDE Feature Request: https://bugs.kde.org/show_bug.cgi?id=474826
Gnome Extension: https://www.omglinux.com/gnome-bedtime-extension-greyscale-mode/