Get new shift cables. The cold affects cables, and it's likely you have damp in them and the cold starts them to freeze.
If they are shifting well at above freezing and aren't dragging, then OK you might not need to replace.
Problem is, the bike is in a garage, and it will be close to, or near freezing by the time you get on it in the morning, and will likely freeze when moving.
I've had frozen cables in the past, and on one occasion, a frozen freewheel - road was frozen so had to push to the main road. By the time I got there, the freewheel pawls had frozen open = no drive.
The answer= if it's going to be a very cold night, bring the bike inside the house until the morning. Allows it to dry out, and no freezing.
The other answer is ride fixed. Only brake cables to worry about freezing.