A historian of the night, at Virginia Tech, twenty years in the depositions.

He kept meeting a phrase he could not place. Not a theory of sleep and not an argument about it — a passing reference, dropped without introduction, in the register of a thing nobody needs told: first sleep. It is in court testimony and in physicians’ manuals and in Chaucer and in prayer books and in a French doctor’s advice on the hour most likely to conceive, and it is in more than two thousand places in a dozen languages, and in not one of them is it explained, because in not one of them was it news.

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.

A. Roger Ekirch, At Day’s Close: Night in Times Past, 2005

People woke after midnight and stayed awake. They talked, prayed, had sex, tended fires, took medicine, interpreted the dream they had just had, and went back down for the second sleep. The references thin out in the late seventeenth century and are gone within a few generations, on a schedule that tracks street lighting.

Two thousand mentions and not one explanation. A practice is only described once it has started to go.