This depends on so many variables which are only known to you that I believe it's impossible to provide you with answers.
- what's the flow temperature? Do you want to change that as well?
- what kind of house is it? (stone/wood/etc.)
- how big is the house?
etc.
My approach would be to simply test it - try start heating at 6am, when do you finally reach 21°C?
Plus, does it make sense to go to 16°C at all?