Trust me, you are not actually at zero brightness. You are the lowest available preset for whatever widget you have used/configured. Some apps that provide brightness Widgets have strict variable levels that they allow you toggle between, others allow you to enter custom values.
It is important to know that you are probably at 7% or so at lowest. Someday you may, like I did, decide to make your own widget with just the preset values you find usefu. If you believe, like I did then and you do now, that zero brightness is simply very dim, you will have a rude awakening when your screen goes black and you cannot remember exactly where the toggle is by touch alone.
Trust me on this one. Or don't and try it, just remember where that widget is placed (I am pretty sure Elixir is where I defined my levels when I did it, but I have seen the capability several places.)