One of the worse things you can do is to dampen down your fire overnight.
Leaving your heater overnight at the minimum air setting with a full firebox generates very little heat and lots of smoke causing pollution outside and inside your home.
With the air intake closed, the fire is starved of oxygen and cannot burn properly.
Unless your heater is specifically designed for continuous operation (burning bright overnight), let it go out overnight. Rely on your home's insulation to hold in enough heat for the night. Spend money on insulation rather than fuel - it is better for the environment and your health.
If you have a new certified heater designed to burn overnight, follow the manufacturers instructions carefully. Don't cram the firebox full. When the last load is put in, run the heater at maximum for half an hour, then adjust it to the overnight setting.