A fiddler, an individual whose incessant interference during the night can cause a predicament the next day. This could be at work i.e. a night worker putting a day worker up shit creek without a paddle because of his meddling during the late shift. A second application could be made in a domestic situation where a family member raids the fridge late at night leaving a mess to be cleared up the next morning. Midnight Meddlers invariably do not sort out their own mess.
Sam: "What the hell happened here? It was fine last night"
Rollo: "Must have been the midnight meddlers."