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Sleeping in your car may be illegal in certain places, especially if the individual sleeping has been drinking alcohol. The laws about drinking and driving vary from country to country, and can sometimes rely on a policeman's judgement regarding whether they think you're intending on driving before fully sobering up. The best way to avoid potential problems is not to drink at all - however, if it is unavoidable, do not put the keys in the ignition, and certainly do not start the car, not even to run the heater. Instead, put the keys in the glove box - and be sure not to sleep in the driver's seat. Be aware that if the car is parked on public roads, the police or other emergency services may need to ask you to move it, which can make life interesting.
I once slept in a Renault Fuego in a North Wales lay-by. I was parked underneath a sign saying 'No overnight sleeping in cars' and was woken by two of the boys in blue. I gave them the (amazingly truthful) explanation that I had 'just driven there from Somerset and was too tired to drive any further. But,' I added, 'if you insist that I drive while unfit to do so, I will move on, but I'll want an assurance in writing that I won't be prosecuted for "driving while unfit" before I do.' After a muttered conversation between them, and a quick call to control which ended with the one who had called back shrugging his shoulders to the other one, they decided that national law overruled the local bylaws, so I was allowed to stay there until 10am the next morning.
Furthermore, in some places sleeping in a car may be a dangerous practice, as there could be a possibility of being mugged or even killed. If you can find a safe place, however, sleeping in your car can be an extremely useful option when making road trips. On a weekend excursion it can allow you to save money that would otherwise be used to pay for a hotel room, and it can also remove the possibility of driving unsafely after a good night on the town. It may also be necessary to sleep in one's car for a variety of unforeseen reasons - when broken down and awaiting rescue during daylight hours perhaps, when you're so completely lost you can't even find a town (let alone a B&B), or when you've locked yourself out of the house (or someone else has locked you out), or when very bad weather has forced you to stop, or, in extreme circumstances, when you don't have anywhere else to sleep.
Don't Expect a Good Night's Sleep
Despite the fact that it is possible to sleep in a car, you must realise that an automobile is not designed for sleeping in. Rather, it is designed for transportation. Thus, most car manufacturers have not designed cars with sleeping in mind. For example, the default position of a car seat is upright (preferred for driving), rather than lying down (preferred for sleeping). This stems from the fact that attempting to drive and sleep at the same time is a rather dangerous practice. Therefore, when you do decide to sleep in your car, please make sure that the car is parked. Accepting the fact that your car is not a conducive environment for sleeping in is something that must be done before any attempt to get a good night's sleep in it. If this is not heeded, you will be very disappointed.
There are certain tips that will help you get a better night's sleep in a car:
In times of bad weather, or troubled relationships, it is a good idea to keep a few supplies in your car. A blanket or sleeping bag (even in summer it can be very cold in a car at night), some water (enough for the engine and yourself), a shovel, and some old carpet. The shovel and carpet are for digging yourself out of snow (or mud), and the carpet can also be used for insulation if necessary. Keeping a set of fuses and bulbs handy can help you to avoid nights in cold cars, as long as you know what you're doing - be sure to keep the manual in the car too!
If You're Intending on Having a Tipple...
If you are attempting to sleep in your car, you may have spent the better part of the night drinking alcoholic beverages. Attempting to adjust the seat of a car from the upright to lying position after several drinks can range anywhere from challenging to impossible. Therefore, in order to avoid the frustration of seat adjustment upon returning to your car, it is a good idea to adjust the car seats to a position conducive for sleep before you leave it. The back seats of some estate cars can be flattened completely, giving a roomy sleeping area.
Select a Spacious Car to Sleep In
If you attempt to sleep in a vehicle such a Ford Ka, you are unlikely to get a good night's sleep. Such a vehicle is small and has thin doors that do not keep the heat in very well. A larger vehicle with a more spacious interior, such as the Ford Cortina, will offer superior comfort.
Park the Car in a Warm Place
Due to the fact that the Earth rotates on its axis, the location where your car is parked will at some point face away from the sun, making it rather cold and dark. As most people sleep at this time, you will be sleeping in a vehicle that may get rather cold on the inside. Therefore, it is preferable to park the car in a building such as a parking garage, which is likely to be warmer than parking it near a river, for instance. Ensuring that you have a blanket in your car will also help ward off the cold.
If you see any signs saying that overnight camping or overnight parking is not allowed, don't park there.
Under no circumstances, no matter how much it's raining, how much you've drunk or how unable you are to get the door open should you ever, ever go to sleep under a car.
Remember Where Your Car is Parked
This is not an easy task, especially if you decided to drink alcoholic beverages before attempting to sleep in your car. This is doubly true if you are in a city that you are unfamiliar with. So, in order to avoid wandering around at 5am on the streets of Luxembourg searching for your vehicle-cum-bedroom, you need to remember where your car is parked. A good way to do this is to get a map from the city's tourist information centre.
Another technique for locating your car is to take a picture of it with a digital camera once you have parked. However, this technique may also fail - as you carry the digital camera with you throughout the night, you will inevitably take more pictures and, probably in some state of mind altered by potent beverages, decided to delete the silly picture of your car in order to make room for a more juvenile picture.
Overall, sleeping in a car is not a bad idea. It provides an (admittedly uncomfortable) alternative to sleeping in an overpriced motel or hotel, or under a hedge.