In a few states—like California—you can be convicted of driving under the influence (DUI) only if there's proof that you were actually driving. In these states, merely sleeping in your car while drunk isn't enough for a DUI conviction.
However, in the majority of states—Arkansas, Nebraska, and Washington included—being caught sleeping drunk behind the wheel can lead to a DUI conviction. That's because the DUI laws of most states prohibit "operating" or "being in actual physical" control of a vehicle while under the influence. So, driving is sufficient but not required for a conviction.
States differ on exactly how they define operating or being in actual physical control. But the aim of these laws is the same: To prevent drunk driving before it begins. After all, a motorist who's drunk in the front seat with the keys in the ignition is just a few quick motions away from putting the car in gear and driving away.
A jury will generally look at all the circumstances of a case in determining whether an impaired motorist was operating or in actual physical control of the vehicle. Typically, the closer the driver was to being able to set the car in motion, the more likely the jury is to convict. So, an impaired motorist found sleeping in the back seat with the motor shut off might have a good shot at beating the charge. But a drunk motorist who's caught draped over the steering wheel with the car running is more likely to be convicted.