“People don’t want to talk about it. It’s a dirty little secret,” says Lee Crespi, a New York City-based couples therapist. “There are people who say sleeping apart is not good because it fosters distance, but I think you can argue both ways. People do, in fact, sleep more soundly when they sleep alone.”

Years ago during a dinner with friends, the topic turned to a married couple that not only slept in different beds, but different rooms. They were parents, they loved each other, and that was the arrangement that clicked. My wife and I agreed that would not work for us, that it was important to sleep in the same bed no matter the challenge. One of the perks of being in a relationship is waking up next to someone. Also, more practically, we lived in Manhattan and could not afford separate bedrooms.

Sleep, much like running a marathon or chewing food, is a solitary activity. We physically lie next to each other, but we sleep alone. So why did this custom originate? According to Virginia Tech professor Roger Ekirch, an historian and author of the book At Day’s Close: Night in Times Past, there used to be a financial incentive to sleeping together, as recently as the 1800s.

“Even livestock often resided under the same roof, because there was no other structure to put them in, and they generated welcome warmth. Among the lower classes in preindustrial Europe, it was customary for an entire family to sleep in the same bed—typically the costliest item of furniture—if not to ‘pig’ together on a straw pile,” Ekirch says. “Genteel couples, for greater comfort, occasionally slept apart, especially when a spouse was ill.”

Television affirms this, but only partially. Charles and Caroline Ingalls shared a bed in the late 1800’s on Little House on the Prairie, the cabin of which, true to the program’s title, was far too small for a family of six. But Robert and Cora Crawley, who certainly had the funds to support separate snoozing quarters at Downton Abbey circa the early 1900’s, still chose to toss and turn on the same mattress.

It appears our history of bunking together runs much deeper than just financial necessity. We human beings are also scared of the dark.

“Night, man’s first necessary evil, inspired widespread fear before the Industrial Revolution,” Ekirch says. “Never did families feel more vulnerable than when they retired at night. Bedmates afforded a strong sense of security, given the prevalence of perils, real and imagined—from thieves and arsonists to ghosts, witches, and the prince of darkness himself.”

Borrowing from another television genre, horror movie fans know the safety of sleeping with a partner, under the covers, with the door shut. It’s when one bedfellow goes on solitary reconnaissance to investigate a midnight noise that the chainsaw-wielding madman leaps out of the shadows. In modern times, sleeping together has less to do with being afraid of witches or burglars, but rather the fear of a different, social demon.