UPDATE ON FIRST SLEEP/SECOND SLEEP. I posted here about first sleep/second sleep in response to Annalisa’s observation that this was a hobby horse that I talked a lot more about than I posted about. My nephew Andrew has now sent me a link to an article by Benjamin Reiss on the New York magazine website. Reiss credits an essay in 2001 (less than 20 years ago) by Roger Ekirch with first calling attention to the fact that the ideal of eight hours of unbroken sleep had developed only in the 19th century with the spread of artificial light. Since 2001, there has been a lot of debate about whether any particular sleep pattern is more natural for Homo sapiens than others. It looks like the issue is still unsettled.
A poetical point: The interval between first and second sleep is sometimes called “the watching”.