Sleeping in one's car may be illegal in some places, especially if the individual sleeping has been drinking. The laws about drinking and driving vary from country to country and could rely on a policeman's judgement of whether they think you may be intending to drive before fully sobering up. The best way to avoid this is to avoid drinking at all, however, if it is unavoidable, do not put the keys in the ignition, and certainly do not start the car, even to run the heater. Don't sleep in the driver's seat, and put the keys in the glovebox. Be aware that if parked on public roads the police or other emergency services may need to ask you to move your car, which can make life interesting.

I once slept in a Renault Fuego in a North Wales layby. 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.

Further, in some geographic locations, sleeping in one's car may be a dangerous practice as there is a possibility of being mugged or even killed. However, if you can find a safe place, sleeping in one's car is a useful tool for one's 'bag of tricks' on road trips.

Sleeping in one's car on a weekend trip can allow one to save money that would be use to pay for a hotel room and avoids the possibility of unsafe driving after a good night on the town. It may also be necessary when broken down and awaiting rescue during daylight hours, you're so completely lost you can't even find a town, let alone a B&B, you've locked yourself out of the house (or someone else has locked you out), very bad weather has forced you to stop or, in extreme cicumstances, you don't have anywhere else to sleep.

There are some certain tips that will help one get a better night's sleep in their car.

Supplies

In times of bad weather, or troubled relationships, it's a good ideas 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 can help to avoid nights in cold cars, as long as you know what you're doing - keep the manual in the car too.

Don’t Expect a Good Night’s Sleep

Despite the fact that it is possible to sleep in one's car, one must realize that an automobile is not designed for sleeping. 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 mere 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 one's car is not a conducive environment for sleeping is something that one must accept before attempting to get a good night's sleep in a car. If this is not heeded, you will be very disappointed.

Put the Seats in the Reclining Position Before Leaving the Car if You're Intending to have a Tipple

If you are attempting to sleep in your car, chances are that 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 as you wander back 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 spacious area to sleep in.

Select a Spacious Car to Sleep In

If you attempt to sleep in a vehicle such a Ford Ka, you will most likely not 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, such as the Ford Cortina, offers superior comfort and a spacious interior.

Park the Car in a Warm Place

Due to the fact that the Earth rotates on its axis, and for several hours a day (the actual amount of time depends on what part of the world you are in) the location on Earth upon where your car is parked will 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 warmer than parking it near a river, for instance. Ensuring that you have a blanket in your car ahead of time will also help ward off the cold.

If you see any signs saying overnight camping, or overnight stays 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 5 am on the streets of Luxembourg searching for your vehicle/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 after it is parked with a digital camera. 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 some more juvenile picture.

Overall, sleeping in one’s car is not a bad idea. It provides an (addmittedly uncomfortable) alternative to sleeping in an overpriced motel or hotel, or under a hedge.