GDM does not return to sleep mode when my session is locked. Normally, when my session is not locked, my laptop goes to sleep mode after 15 minutes, but if GDM is active, rather than my unlocked session, it just doesn't...
So for instance, if I just hit a key to watch the hour or whatever, it wakes up and stays awaken, and then drains all the battery if I forget to re-suspend it manually...
How can I change that behaviour?
I have an Ubuntu 18.04 install, upgraded from 17.10.
It seems it could come from a missing configuration in /etc/systemd/logind.conf, especially
IdleActionSec which could be set to 'suspend' and '15min' for instance. I haven't changed anything yet, if someone knows better about logind, I am taking your advices !!