According to bug 1193222, comment #13, I need to install powerd in order to get the screen to sleep on inactivity if I use Mir (the default for Saucy) as display manager.
But if I try to install powerd, modemmanager gets removed:
till@till-twist:~$ sudo apt-get install powerd
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following extra packages will be installed:
libhardware2 libhybris libmedia1 libubuntu-application-api1 ofono
The following packages will be REMOVED:
The following NEW packages will be installed:
libhardware2 libhybris libmedia1 libubuntu-application-api1 ofono powerd
0 upgraded, 6 newly installed, 1 to remove and 0 not upgraded.
Need to get 854 kB of archives.
After this operation, 1.237 kB of additional disk space will be used.
Do you want to continue [Y/n]? n
This means that I can either have the feature of the screen going to sleep or cellular data support, not both.