Common situation
A gas heater and a stuffy room: check this first
A room that feels stale with a gas or other fuel-burning heater running is a safety question before it's a comfort one. Start with the heater.
What it usually points to
An unflued gas heater puts moisture and combustion gases into the room, and an open-flued one can spill them back in when exhaust fans run. Stale air overnight usually means too little fresh air for the people in the room.
What to check and try first
- If the heater is unflued or open-flued, don't run exhaust fans or the rangehood while it's on, and have it serviced at least every two years. If anyone gets headaches, dizziness, nausea or drowsiness when it runs, get everyone into fresh air, turn the heater off if you can do it quickly, and seek medical help.
- Leave the window slightly open overnight, or the bedroom door open if that's warmer. Open it wide for a few minutes when you get up.
These steps assume only: room feels stuffy or stale, especially overnight; a gas or other fuel-burning heater used indoors. Your home may be different. Add what else is going on and you'll get steps for your situation.
Check my home with these answers
What would change this
If you wake with headaches, or a gas heater runs in the room overnight, stop and get it checked. That's a safety question, not a comfort one. If smells persist with the rangehood venting outside, get any gas appliances checked and serviced. If the home has an open-flued gas heater, don't run exhaust fans or the rangehood while it's on: they can pull carbon monoxide back into the room. Have gas heaters serviced at least every two years.