It’s the electricity. Without artificial light in the form of streetlights, electric night lights, and the glow from various appliances and gadgets, even frequent wakers among babies would sleep longer.
Thus, the conflicting advice on how to get an infant to “sleep better” only serves to divide and instill needless guilt about not having a “good sleeper” when the problem is something much bigger and not easily erased with better planning. One of the reasons co-sleeping is more effective at extending infant sleep anecdotally is because master bedrooms tend to be the darkest room in the house at night. A lot of infants are put in cribs with glowing monitors and/or night lights, which is mostly not the case when the baby is in a cosleeper or bedsharing.
The recent trend of fretting about nighttime computer use and “blue light” from laptops and smartphones and tablets affecting adult sleep is the tip of a huge iceberg of modernity.
Our foremothers got more sleep at night even with frequent-waking infants because even a pretty frequent waker just seems to drop some wakeups in (relatively) natural night darkness. And it is really hard to get rid of all the sources of light. We live in a part of the country where people are wired for generators because of the rural setting and it’s still lit up to a high degree at night, even homes nowhere near the street lights. And it is quite shocking to realize how much stuff glows at night in the kitchen, the living room, the bathroom, etc.
I’ve seen wakings go from every 90 minutes to every three hours just by putting the infant in approximately natural darkness. However, it is very difficult to maintain in any kind of standard American household setting, even among homeschooling conservatives who don’t have televisions (but definitely have some iDevices lurking).
Now there’s a practical project I’d love to see some technically adept conservatives tackle– housing design so that natural darkness can be preserved during nighttime. There are a number of possible strategies, and imagine being able to live in such a home, or have your own home modified to have both modern electric lighting and the ability to get a mostly natural level of darkness at night even in the city. The health benefits to mothers alone would probably increase the old TFR by a couple tenths of a point. Fertility can sometimes be delayed partly because of the sleep deprivation that has become the unfortunate norm for modern SAHMs.