You're fine. Stop insisting that the vents on a car automatically close when the car is shut off. THEY DON'T.
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Answer. The easy answer to this question is that you will never run out of air. You will just exchange the breath that you breathe in with the breath that you breathe out.
Parking in a garage, whether heated or not, will keep your car warmer than parking it outside. Even a carport keeps a car several degrees warmer than one parked outside with no protection. Keeping it warm overnight.
If you are the driver in charge of the vehicle it is illegal to sleep in it 'on the public highway' this includes laybys! Even if you take keys out of ignition, sit/lie in a place that is obviously not the driving seat, it is still not legal.