Brightness settings on my Nexus 5

I have a nexus 5 and it has generally no problems with the brightness levels. But when I am in a totally darkened place, the autobrightness of the phone doesn't take it to the lowest brighness levels, intead it takes it to the second or third lowest levels. Lowest brightness is never achienved. Can an expert tell me what is wrong in this case and what can be done to solve this issue?
