dead of night

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.