Can you legally sleep in your car or a rest area overnight?

It genuinely depends on where you are — rules are set by states and local councils, so there's no single national answer. Many highway rest areas are designed for exactly this and allow a fatigue stop (often signed with a time limit); others, and most urban streets and beachfront car parks, ban overnight sleeping and you can be moved on or fined. As a rule of thumb: highway rest areas for fatigue are usually fine and actively encouraged for safety, while town and suburban parking usually isn't. Read the signs, use travel apps to find sanctioned overnight stops, and never ignore tiredness — a fatigue stop is always safer than pushing on.