Yes you can. The official rules in California state that you can stay for a maximum of eight (8) hours at a rest area. Thus, if you happen to stay from late night to morning, then you have effectively stayed overnight in a California Rest Area. Otherwise, the State does not prohibit you from sleeping in your vehicle during your stay.
Can You Stay Overnight in California Rest Areas?
As you don’t stay longer than eight (8) hours, you can stay overnight at a California Rest Area. The California Department of Transportation (CalTrans) does not prohibit visitors from sleeping in their vehicles. They do publish a list of rules for using rest areas, but those rules do not stop you from sleeping overnight, as long you don’t exceed the maximum stay. Read the full California Rest Area Rules.
Camping, on the other hand, is prohibited. But there is a difference between camping and overnight parking. As long as you don’t roll out your awning, set up outdoor camping equipment, or watch television outside, you should be fine.
What if I Need to Stay Longer Than Eight Hours?
Then stay longer.
The fact is that most RVers and car campers do stay longer than eight hours when overnighting at rest areas. It is rare for California Highway Patrol officers to cite people for staying beyond the maximum time limit. Law enforcement officers do routinely stop at rest areas, but as long you leave next morning they usually don’t knock on your door. They’d rather you get plenty of sleep than cause an accident later on.
If you’re in a rest area that has plenty of available parking spaces for trucks and RVs during the late night hours, then it’s likely you could stay 24 hours or more and not be bothered. Meanwhile, rest areas that usually become full overnight are more frequently patrolled. For example, Weed Rest Area along I-5 in Northern California is extremely busy, and is frequently patrolled by CHP.
The best piece of advice for staying longer than eight hours at a rest area, is to not draw attention to yourself. That is, don’t make excessive noise, don’t loiter outside, dispose of all trash into trash receptacles, park only in marked parking spaces, and keep your dog on a leash.