Throughout most cultures for a large part of human history, people've slept in a polyphasic cycle called biphasic sleep. Up until the industrial era, sleeping biphasically-- that is, two four-hour segments of sleep seperated by two or three hours of wakefulness-- was the norm. [1]
But with industrialization and the electrical light, how we sleep shifted to a way that's rather unnatural. To keep up with earlier hours and longer commute times, people had to get up earlier and earlier-- and in order to get enough sleep, the gap of wakefulness between cores disappeared. For free time, people began to stay up later. As if to compensate for a lost time in the middle of the night, a ``night-life'' before sleep became the norm. [2]
In 1600, terms like ``first sleep'' and ``second sleep'' were regular in everything from diaries to court documents. By 1850, they had all but died. [3]