Is It Safe to Sleep in Your Car with the Windows Closed?


Man Sleeping in a Truck

My job frequently has me on the road traveling for long distances, sometimes out of state. More than once I have found myself too tired to drive and wanting to pull over and sleep at a rest area. I have often wondered if doing this with the windows up is safe, so I did some research and these are my findings.

Sleeping in your car with the windows closed is generally safe, though external factors can make it hazardous. Vehicles aren’t airtight, and a mid-size car holds 3,000-4,000 liters of air, so oxygen-deprivation is not a concern. What will affect your safety is having the engine on and the location of the vehicle.

There are a few considerations you should make when deciding whether or not to sleep in your vehicle. Let’s take a look at them in greater detail.

There Is Enough Oxygen in Your Vehicle to Keep You Alive

The air we breathe is comprised of 78% nitrogen, 21% oxygen, with argon, carbon monoxide, and various other gases making up the remainder. When we breathe in the air we only use around 5-6% of its oxygen content and exhale the remaining 15%. This is why mouth-to-mouth in CPR is effective at oxygenating the patient.

Therefore, in your car, you can breathe the same air 3-4 times until it doesn’t have a sufficient oxygen concentration to be effective. If we consider that an average adult inhales 7-8 liters of air a minute, and the car has 1000’s of liters of air, there is sufficient oxygen in your car to keep you breathing comfortably for several hours while you sleep.

In fact, a study has been done specifically to find out if being in a car too long is actually an issue. In short, they found that even when a car is parked the air is completely replaced within hours even with zero human intervention or built-in ventilation being employed.

The study did find that if about 9 people or more are in the average car, then oxygen could theoretically be depleted before it could be sufficiently replaced. This is not a likely scenario. Vehicles that hold that many people would have much more air than the average car. You get the point here. It’s not really something to be concerned with.

Do You Have to Crack a Window When Sleeping in Your Car?

You do not need to crack the window of your vehicle while sleeping. Not only is air constantly circulating, there is generally enough air inside the car to sustain someone for a long time.

Despite this, you may choose to crack a window to aid in circulation and this should be considered if you are not alone in the vehicle and/or you expect to be sleeping for more than a few hours.

Even so, for personal safety reasons, it is best to only open the window as far as necessary. You don’t want the gap to be big enough for a third party to have access to the internal compartment of your car.

To put it in perspective- If you are concerned about sleeping in your car, do note that long-haul truck drivers have been sleeping in their vehicles for decades. They do not fear suffocation, crime, or the pressures of climate as the lay in their sleeper cab. There’s no reason for you to either. However, there are factors to consider to give you the best sleep possible in your car.

Vehicles Aren’t Airtight

While some cars are built a bit tighter than others, no vehicle made for the general population is completely airtight. I mean, think about it. Not only is it extremely difficult to seal the passenger compartment it’s also a matter of liability. No car manufacturer wants to be held responsible for someone suffocating inside their vehicles.

In fact, cars and trucks are designed to allow airflow to come into the vehicle from the front. Put your hand in front of your AC vents. Even when the AC and Heat are off, you will still be able to feel a bit of air coming through. This is constantly being circulated and freshening up the inside. Even when sitting still, air can still come in and go out of the vehicle.

Will I Get Carbon Monoxide Poisoning If I Sleep with the Car Running?

Car fumes contain several poisonous components which can be life-threatening if inhaled in large quantities. The biggest concern of these in relation to sleeping in your car is carbon monoxide.

Annually, 400 Americans deaths are attributed to carbon monoxide poisoning and this is the biggest threat to your life when sleeping with your engine on. The dangers are amplified if your vehicle is also in an enclosed space. Without proper ventilation, fumes from the exhaust will enter your car and mix into the air you breathe.

A Word of Caution: As carbon monoxide is a colorless, odorless gas, and an early symptom is fatigue, you won’t realize that you are being poisoned until it’s too late.

Avoiding Carbon Monoxide Poisoning in Your Car

To avoid being exposed to carbon monoxide, park your car in an open space and only run your engine for short periods to provide heated or cooled air that you need. In the same vein, don’t park near other vehicles that have their engines running.

You are also at a higher risk of carbon monoxide poisoning if there is a fault in your exhaust system, so it’s good practice to get your car checked regularly for faults. If sleeping in your car is a long term plan, or you like to be extra cautious, you can purchase a carbon monoxide detector, like this really good one found on Amazon, to give you peace of mind.

If your engine isn’t running, and your vehicle is otherwise in good condition, the chances of developing carbon monoxide poisoning while you sleep are extremely low.

Man Is Your Biggest Safety Issue When Sleeping in Your Car

Unfortunately, one of the greatest concerns about sleeping in your car is the vulnerability you can feel at the hand of your fellow man.

Ideally, you’ve managed to pull over into a designated rest area, with like-minded sleepy travelers. That sense of community and security is priceless. However, if that isn’t available, you should pick somewhere well lit to discourage nefarious activities close to you.

You also want to maximize your visibility if you stopped to sleep close to a road, so other vehicles don’t accidentally hit you as they drive by. This is especially important if you have stopped to sleep through inclement weather, or at night when visibility is poor.

Additional safety precautions are to keep the doors locked and the windows barely cracked if at all. If you can, an option is to park the driver’s side close to a wall or a tree for additional protection, as this will make car-jacking more difficult. I also recommend carrying “Key Chain” pepper spray (Click to see Amazon Listing) if you don’t have some other form of protection.

Climate Considerations

Weather is a key reason why you may choose to sleep in your car. You might unexpectedly encounter a wintery storm in the mountains, or a flash flood in the desert, and it becomes necessary to take shelter in your vehicle for hours. In both of these circumstances, the weather will require you to keep those windows closed.

As you wait for the weather to pass, you can feel comfortable that sleeping in your car is a safer option than tackling the hazardous conditions.

Beware of Heat Exhaustion

If you stop to sleep in extreme heat, extra precautions need to be taken to prevent overheating which can lead to death. In this instance, cracking a window and using your air conditioning occasionally is probably essential to getting some necessary airflow.

Here are some other tips to beat the heat:

  • If you have them, use a fan, like this clip-on air vent fan found on Amazon.
  • If you have a moonroof, open it to let the hot air out.
  • If you have a windshield reflector, deploy it.
  • Drink plenty of fluids. It’s important to keep your body cool and hydrated throughout the night. I keep a 24 pack of water in the back of my car at all times during my travels, just in case.

Dealing with Cold Weather

Conversely, you might be spending the night in your car to avoid the freezing conditions outside. Perhaps counterintuitively, you will want to crack a window in this scenario. Your relative heat to the cool air inside your car will produce moisture and a slightly cracked window will keep that moisture at bay and this will keep you warmer.

If you can, get inside a sleeping bag. I recommend getting one that is rated “4-season”, like this one.

Or, just put on additional clothes to keep your core temperature up. Always keep warm clothes and blankets in your trunk. As stated previously, use your car heating system sparingly. A couple of hours with your engine on in an idle car can easily use a gallon of gas.

Final Thoughts

Sleeping in your car, while usually uncomfortable, is not especially dangerous. As long as you take the necessary precautions and make sure local laws allow you to sleep in a vehicle, there is nothing wrong with getting some shut-eye behind the wheel.

Trust me, it’s much better to risk sleeping in a car than to take a chance of falling asleep at the wheel.

Thanks for reading!

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Related Questions

Is it legal to sleep in your car at Walmart? It is almost always legal to sleep in a Walmart parking lot. However, you may want to go inside and purchase a few things before settling in for the night in case they have a local policy to only allow paying customers to sleep there.

Can you get a ticket for sleeping in your car? It is legal to sleep in your car in most areas, however, many areas (cities especially) have cracked down on the practice over the past few decades. You will want to check local laws before settling in for the night behind the wheel. If caught asleep, you will usually just get a warning. Tickets are generally reserved for repeat offenders.

Jim James

Jim James spent most of his childhood outdoors fishing on lakes in his area. Due to his scouting background, he has always been interested in survival, camping, and the outdoors in general. Jim is a best-selling author and has a degree in History, Anthropology, and Music. He lives with his family in Charlotte, NC.

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