Overnight implies that you park there before night falls and leave after morning arrives. In Winter, that might be 4pm until 10am, so parking 6pm till 9am is just part of the night, not overnight. In Summer, perhaps 11pm until 4am?
I would say that staying until 1am or 2am is probably just parking late into the evening. Parking from 2am until mid-morning could just be a very early arrival.
But if there is a regulation that prohibits overnight parking (very rare: most of these signs have no legal basis) that regulation will define what is meant by "overnight". If it is not defined, the legal interpretation a court might make would rely on the natural meaning of the words.
Note that camping and parking are very different things. You may think you are building a camper, but the law says it is a private car with a body type of 'motor caravan'