There is no national answer to whether you can sleep at a rest area, and the reason is not that states disagree about the number of hours. It is that they do not prohibit the same activity. One state bans the parking. One bans the camping and says nothing about hours. One bans the camping and puts a clock on everything else. Read the verb, not the headline.
Three states, all checked today, make the point.
Maryland bans the parking
COMAR 11.04.07.11, verified today, is one sentence: “Camping and overnight parking are prohibited.”
That is the strictest of the three constructions, because the violation is the vehicle being there. It does not matter whether you set anything up, whether you are asleep, or how quiet you were. There is no hour limit in the regulation and no exception for sleeping in a legally parked vehicle, which some states do write in.
Utah bans the camping and permits the long stop
UDOT’s rest areas page, verified today: “All rest areas are posted for no overnight camping. However, extended stays are permitted and are monitored by the on-site staff and the Highway Patrol.”
That second sentence is unusual and it is worth reading twice. Utah is telling you the long stop is allowed and that somebody is watching it. What Utah does not do is define an extended stay or publish a maximum number of hours, so we are not going to invent one. If you want a number for Utah, the posted sign at the site is where it will be, and it beats this page.
Oregon runs a clock
OAR 734-030-0010(18), verified today, prohibits “Setting up a tent or other structure, camping, or remaining in a Rest Area for more than 12 hours within any 24-hour period.”
Oregon puts camping and the clock in the same clause. Twelve hours in any 24 is generous by rest area standards, and it is a real number you can plan around. Note the wording though: the count runs “within any 24-hour period,” which is a rolling window rather than a per-night allowance.
Why the aggregator lists get this wrong
A list titled “states where you can sleep in your car” has to turn all of that into a yes or a no. Maryland becomes a no, which is right. Utah becomes a yes, which loses the fact that camping is prohibited and the hours are undefined. Oregon becomes a yes, which loses the 12 hour ceiling. What gets flattened is the part that decides whether somebody knocks on your window.
The rule you are subject to is the one written by the agency that runs that rest area, and the sign at the entrance reflects it more currently than any website, including this one. Read it before you settle in.
Our state pages carry the source and the last-verified date for each: Maryland, Utah, Oregon. If the distinction between parking and camping is the part you want worked through properly, start with our camping versus overnight parking guide.