[aomurphy]

There's a very active historical debate about whether a schedule like this, often called "biphasic sleep", was more common in pre-industrial societies. There's a historian called Roger Ekirch who thinks it was, starting in his 2004 book "At Day's Close - Night in Times Past". There's a bunch of criticism of it, since the sources are a bit ambiguous, or if he's generalizing from Medieval England (his main focus).