dead sleep
noun1
Usually A particularly sound sleep
2
Usually The 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: