When driving after dark, the automatic dimming rearview mirror reduces the glare from headlights behind you, based in inputs from the mirror sensor. This feature is always active.
The auto dimming function cancels when the shift position is in R.
Honda Civic Service Manual. Headlight Adjustment ('12-'13, '14: 4-door)
1.
Headlight - Adjustment
Headlights become very hot during use; do not touch them or any attaching
hardware immediately after they have been turned off.
Before adjusting the headlights:
...