small hours

From the small number used to identify such hours (e.g. 1 o'clock, 2 o'clock).

small hours pl (plural only)

1. The very early morning, just after midnight, when most people are asleep.
   • 1962 April, J. N. Faulkner, “Summer Saturday at Waterloo”, in Modern Railways, page 265:

     Then follows a quiet period during the small hours, marked by the departure of the other newspaper trains and some van trains, and by the arrival of the mail train from Weymouth.

   • 2017 June 26, Alexis Petridis, “Glastonbury 2017 verdict: Radiohead, Foo Fighters, Lorde, Stormzy and more”, in the Guardian‎:

     ...the writer conjured up a dystopian fantasy more berserk than anything you might find yourself listening to in the small hours at the Stone Circle.

Almost always used with the definite article (the small hours), or occasionally with a demonstrative adjective (these small hours, those small hours).

• (very early morning): wee hours, wee small hours, ungodly hours; see also Thesaurus:early morning