1. Age Factor: I remember when I was younger I required less sleep (or so it seemed). Consider that women used to marry and have children at a younger age a century ago (if not less).
2. Schedule: If we're talking about natural sleeping habits (for parents and children), it would seem appropriate to talk about what goes on during the day... Our daily chores may not be as physical as they were for our great grandmother's, but is it possible our schedules are too full with extra curricular activities?? Perhaps by simplifying our schedules we could manage the basics with the amount of sleep we are getting... instead of being exhausted and maxed out?
3. Health Problems: there is an entire field of research dedicated to sleep problems... some of it relating to babies/children. I think there are a lot of babies/children with sleep issues that are related to health problems (like the mom's who report their child hasn't slept longer than 45 min. at one time in 18 months or longer). For those extreme sleep deprived families, it makes sense to me that there would be a medical reason for it... especially since sleep is so important to health and growth/development. But I know each person has different experiences.
I know a woman who has 9 children, and she once joked that she had been changing diapers for 3 decades! I imagine that yes, she experienced sleeplessness along with those years. Maybe she didn't need as much sleep as some, but I know coffee was a regular staple at their house.
My SIL has three kids now ages 7, 4, & 2... she considers exhaustion to be part of life.
For us, we do what we can to get sleep when we can. DH will sleep in the guest room when he needs to catch up on sleep, and I go to bed early or sleep in when I can... as well as take a nap when I can.
Bottom line, it might be easy to wonder if the pasture is greener on the other side... but I think Cosleeping etc. are natural and the best way to get the most sleep through the night... based on what I have read about sleep. (consider too that there are a variety of ways to cosleep and that each family has to assess their own needs to find a sleeping arrangement that meets their individuality.)