Our ancestors slept differently.
If you went back to the time before the Industrial Revolution, you would find them turn in around 9 or 10 pm and then sleep for about three and a half hours. Then, they would wake up on purpose and stay up for an hour or so. In that time, they would read, eat, clean, and sometimes visit their neighbors who were also up at this time. After about an hour, they would go back to sleep for another three and a half hours. These two segments of sleep were called “first sleep” and “second sleep” and everyone slept this way.
Sleeping in two segments was the way everyone slept centuries ago, but by the early 20th century this way of sleeping was gone. There were two inventions that contributed to this: the lightbulb and the clock. With artificial light, we went to sleep later, which shortened the first sleep. With the clock, we had to get up early to go to the factory for work. So, the second sleep was shortened. After a while, it did not make sense to sleep for two short segments, so we consolidated them to make the way we sleep today.
Today, there are some scientists and historians who believe that sleeping in two parts, that is segmented sleep, is the natural way of sleeping. Details about how our sleep changed and what we can do to get a good night sleep are all spelled out in a new book called The Alchemy of Us.