In cold weather, blankets or a sleeping bag may provide enough warmth. In hot weather, you may need to crack open your windows, but it is only safe to do so if you are not idling the vehicle. If it is too hot or cold for these methods to work, you will have to run your HVAC system in order to sleep.

While it is not strictly necessary for oxygen (as cars are not airtight), it is highly recommended to crack a window 1–2 inches when sleeping in a car to manage air quality and moisture. An average adult consumes about 0.5 liters of oxygen per minute, and while you won't suffocate in a sealed car, carbon dioxide (CO2) buildup and condensation from your breath can lead to morning headaches, grogginess, and damp upholstery. In 2026, safety experts emphasize that you should never leave the engine running while sleeping, as this creates a fatal risk of carbon monoxide poisoning from exhaust leaks. Cracking a window or using a sunroof vent helps circulate fresh air and reduces "fogging" that signals to outsiders that someone is inside. In cold weather, the window helps manage moisture; in hot weather, it is a safety necessity to prevent heatstroke. For security, using window screens or "rain guards" can allow for airflow while keeping the interior private and protected from insects or rain.