...or, rather, how they didn't sleep.
Do you ever wake up in the middle of the night, and find that it takes a couple of hours to drift off again?
If so, it doesn't mean you have insomnia. In fact, your 3rd great-grandparents probably did this every night-- and it appears to be a perfectly natural pattern to humans.
Before the advent electric lighting in the late 19th and early 20th century, nighttime was a far darker and more dangerous world. Really, there wasn't much you could do when night came, because you couldn't see well.
People generally went to sleep when the light faded, which is much earlier than most go to bed today, and got up when it rose. So your ancestors probably went to bed around 8 o'clock, and then woke around 12 or 1 am. For a couple of hours, they would chat, read, pray, make love, and perhaps even visit neighbors (an aside here: I can hardly imagine getting a 1 a.m. knock at the door and hearing, "Yoo-hoo, Karen, you up for a spirited game of Scrabble?")
Then they would go back to sleep for a few more hours until morning.
Light, as it happens, plays a huge role in how and how much we sleep: darkness tells our brains that it's time to sleep, and light tell them that it's time to wake up. So when electric lighting allowed us, for the first time, to have bright illumination at night (when our brains would otherwise be telling us to go to bed), it allowed us to stay up later and do more. In short, it messed with the segmented sleep pattern that most people in the western world were used to. It gradually stopped, and was completely gone by the 1920s.
So if you find yourself waking during the night, instead of tossing around trying to get back to sleep, just do what your ancestors did and find something quiet and relaxing to do for an hour or so. I usually just read or listen to music on my iPod until I find myself getting sleepy again.
Read more on this here.