While it's true that no one wants to live in a house divided, sleeping arrangements are an altogether different matter.

A bed divided is not just the foundation of a happy domestic union, but also our natural preference. Even if, in the interest of keeping things fresh in a relationship, we try to fight it.

After all, who wants to be like their parents, cemented in their age-old roles? Mom on the right. Dad on the left. They might cross that border for a little affection — luckily for you — but when the dust settles, it's back to their respective camps.

A couple under blankets in bed. Left is left and right is right. But sometimes, the twain shall meet. (Photo: Realstock/Shutterstock)

The thing is, no matter how hard you try to keep things fluid, maybe even switching sides every other night, you eventually pick a side — and stick with it.

And why not?

You've got everything just right on your side of the bed. The night lamp, for instance, has become an instinctive reach. You can turn off your alarm without opening your eyes. That half-eaten snack is still waiting patiently for a late-night nibble. And let the crumbs fall where they may — it's your side of the bed!

In time, you've tailored your side of the bed to your specific needs. Why learn new behaviors every other night? It can be jarring to reset your habits just after you've established a system of sorts.

But exactly why we choose a particular side to set up camp in for a third of our lives is a little more complicated. Like evolutionary psychology complicated.

There's surprisingly scant research on the specifics of sleep sides. One major study, however, suggests we position the entire bed based on primal fear.

The University of Munich researchers found that "people prefer sleeping places that allow them to view the entrances to the sleeping room (doors and windows) from a distance while remaining concealed from the entrances themselves."

The fear of predation, even in coddled suburban homes of the 21st century, still looms so large, participants in the study overwhelmingly positioned their bed as far from the door as possible.

But what about divisions in the bed? Do we instinctively pick the side that's farthest from the door — in hopes that intruders will murder our significant other first?

That's an even murkier research subject. Paul C. Rosenblatt, a professor at the University of Minnesota, came up with several factors in his 2012 book, called "Two in a Bed: The Social System of Couple Bed Sharing."

We may, for example, enjoy the morning sun that streams in from the window — more so than our partner who hates the light, craving only darkness. Bathroom proximity, even if it only saves a couple of steps, may also be a factor. For frequent tinklers, those steps add up. Rosenblatt suggests another, somewhat more noble reason for our choice in bed real estate. Men may sleep on the side of the bed nearest the door to guard their partners, even in sleep.

A cat staring through a crack in the door. Fear of intruders likely keeps us as far from the bedroom door as possible. (Photo: Lewis Tse Pui Lung/Shutterstock)

A 2011 poll goes so far as to suggest the left side of the bed is a happier place. For the survey — sponsored by a hotel chain — 3,000 adults were asked about their happiness at home and work, and of course, what side of the bed they slept on.

The happiest and most well-adjusted participants reported that they owned the left side of the bed. The researchers also noted that most of these "generally more cheerful" sleepers refused to swap sides with righties. There's a limit, apparently, even to love.

But it's important to note the poll doesn't make it clear that the left side of the bed made people happier, or if happier people were just drawn to that side.

Still, for the amount of time you spend in bed, why not hedge your bets?

While things are settling in on that new relationship, you might want to stake your claim early on a side of the bed. Don't be a romantic fool and give your partner first dibs. This is, after all, a lifetime commitment. And if there's any debate, you can always tell your partner you're guarding against bedroom intruders — from the door or window, depending on the side on the bed you prefer.

That way you can have your cake. And eat it in bed, too.

Why we're stuck on a particular side of the bed
When it comes to picking a side of the bed, resistance is futile.