I was going to make the same point that Scarlett did-- about bimodal sleep.
Your Ancestors Didn’t Sleep Like You is one famous summary that’s made the rounds on FB and blogs.
The Long and Short of Bimodal Sleep is another good summary, which includes references from Chaucer and the Odyssey.
A lot of times, when I hear ads on the radio about “sleep studies” and “do you find yourself wakeful in the middle of the night” sort of problems, it reminds me of how, historically speaking, people generally didn’t sleep all the way through the night. I also suspect that contributes to why blind people are more likely to be susceptible to non-24-hour sleep cycles– they don’t get the stimulus from artificial lighting that sighted people get, and consequently, are more likely to naturally revert to more “primitive” (pre-electricity, pre-gaslight) rhythms of wakefulness and sleep.
I remember even as late as the 1930’s or 1940’s, I was reading a Dorothy Sayers story, where the servant blackmails the main character because “…I had just woken from my (first sleep? small sleep?) when I looked out the window and happened to see…” So, even within living memory, normal people were still keeping those wakefulness/sleep patterns as part of daily life.
So, I would suspect, in a religious community, your body would adapt pretty easily, as long as you didn’t stay up until midnight playing Farmville or reading your phone or something. In the wintertime in my part of Texas, for example, sunset is around 5:20, and sunrise is around 7:20. So that’s about 14 hours of night. In a higher-latitude place like Boston or New York, you run into a 7:10 sunrise and a 4:15 sunset, which is almost 15 hours’ worth of night. So, if you imagine trying to sleep for 14-15 hours in the preindustrial world…! It would make sense, especially in certain times of the year, to break it up into multiple periods, get a little something done in between during the dark and the quiet, and then wander back to bed.