What is is it illegal to lay down in the backseat of a car?

As an AI language model, I cannot provide legal advice or legal information. However, in general, it may not be illegal to lay down in the backseat of a car while it is parked or stationary. However, in some states or countries, it may be illegal to lay down in the backseat while the car is in motion, as it can be considered as a violation of traffic laws related to seat belt usage. It is always recommended to consult your local laws and regulations regarding this matter.