Dead Of Night Meaning

(idiomatic) Middle of the night.

Example: 1960, P. G. Wodehouse, Jeeves in the Offing, chapter VII:
  I was feeling just as I had felt in the old Malvern House epoch when I used to sneak down to [the schoolmaster]'s study at dead of night in quest of the biscuits he kept there in a tin on his desk, and there came back to me the memory of the occasion when, not letting a twig snap beneath my feet, I had entered his sanctum in pyjamas and a dressing-gown, to find him seated in his chair, tucking into the biscuits himself.
1968, Blackbird by the Beatles
  Blackbird singing in the dead of night.
  Take these broken wings and learn to fly.