Given a travel related situation, and given a specific question about the situation, summarize the question into a title

When landing at night on most planes they say that due to safety regulations the lights will be dimmed and if you want to continue reading you can use your overhead lamp(it could be that this only happens in Europe I am not sure). I wonder what is the reason for this? What safety does dimming the lights add?
Why are plane lights dimmed before landing at night?