Is it possible to reduce the screen brightness below given level? Well if possible does it really reduce the energy consumption.
Yes it is possible to reduce the brightness level below the default screen brightness slider level. You can use, Night Mode app from Play Store.
As mentioned in the app description,
Night Mode's main goal is to reduce the brightness of your screen lower than you can achieve with the default settings.
And this app doesn't require you a root permission.
Does this setting saves battery?
It depends upon the screen you have for your phone. If you have AMOLED screen it will save battery for you.
See this post for more explanation How does reducing brightness of screen increase battery life?