At least half an hour before you go to bed:
- reload your heater with good quality wood
- run it on a high burn rate (high airflow) for 20 minutes
- turn the airflow down but ensure that a visible flame is maintained.
As general advice, it's worth knowing that:
- closing off all air to a fire produces very little heat and a lot of smoke
- you should only turn down the airflow when you have a hot bed of charcoal
- you should never shut the airflow right down
- let the fire burn itself out overnight will produce less smoke than smothering the fire.
Turning the air supply right down does not gain you any advantage because the fire will only smoulder, creating little heat and a lot of smoke.
Other ways to avoid a smouldering fire
Consider alternatives to leaving the fire burning overnight. A portable electric heater, appropriate for the size of the room being heated, switched on with a timer a short time before you wake, will take the chill out of the morning air.