dead sleep

plural dead sleeps

1. uncountableusuallyA particularly sound sleep
2. uncountableusuallyThe first sleep of the night in a biphasic sleep pattern, before the watch.

   Until the close of the early modern era, Western Europeans on most evenings experienced two major intervals of sleep bridged by up to an hour or more of quiet wakefulness. […] The initial interval of slumber was usually referred to as “first sleep,” or, less often, “first nap” or “dead sleep.”

   2001 April, A. Roger Ekirch, “Sleep We Have Lost: Pre-Industrial Slumber in the British Isles”, in The American Historical Review, volume 106, number 2, American Historical Association, page 364

   Synonyms: first sleep