Sleep under blankets. Blankets are common, but not universal, to humans during sleep, at least in the modern day.
But historically, the effort involved in weaving large sheets put blankets at much too high a price point for most to afford. From the linen bedsheets of Egypt around 3500 B.C. to wool sheets during the Roman empire straight through to cotton in medieval Europe, bed coverings were for the wealthy.
By the Early Modern period in Europe, which followed the Middle Ages, production had increased enough so that more middle-class people could afford bedding, though not easily. “The bed, throughout Western Europe at this time, was the most expensive item in the house,” says Roger Ekirch, a historian at Virginia Tech who has written extensively about sleep. “It was the first major item that a newly married couple, if they had the wherewithal, would invest in.” The bed and bedding could make up about a third of the total value of an entire household’s possessions, which explains why bedsheets frequently showed up in wills.
In place of blankets and sheets, other sources of heat were common at night, usually from multiple people sharing a bed, or often livestock.
Today, there’s minimal anthropological work about bedding around the world. The best is a 2002 paper by Carol Worthman and Melissa Melby of Emory University, who compiled a study of sleeping arrangements in different parts of the world. “Recognition of the paucity of anthropological work on sleep is galvanizing: a significant domain of human behavior that claims a third of daily life remains largely overlooked by a discipline dedicated to the holistic study of the human condition,” they wrote. This passes for outrage in an academic paper.
The paper looked into some foraging and non-foraging peoples who live in hot climates near the equator, and found that only the nomadic foragers regularly sleep without bed coverings. Everyone else uses some form of covering, whether that’s plant matter or woven fabric, even in central Africa and Papua New Guinea, both tropical climates. Much more common than sheets or blankets are some form of padding; basically nobody sleeps simply on the ground as a matter of course.
As one more example of the goodness of blankets, there has also been a decent amount of research about the calming effect of weighted blankets, which can weigh up to 30 pounds. Studies indicate that they can curb anxiety and even be used in the treatment of autism.
About 60 to 90 minutes before a usual bedtime, the body starts losing core temperature. There’s a physiological explanation for that: when the body is heated, we feel more alert. And conversely, when the body cools down, we tend to feel sleepier. Cooler internal body temperatures are correlated with a rise in melatonin, a hormone that induces sleepiness. A bunch of doctors tested this out by making people wear skinsuits—they kind of look like cycling outfits—that dropped their body temperature just a touch, one or two degrees Fahrenheit, to see if they’d sleep better. They did.
Your body’s ability to regulate its own heat gets way more complicated at night, though. Say you sleep for eight hours each night. In the first four hours, plus the hour or so before you fall asleep, your body temperature will drop a bit, from around 98 degrees Fahrenheit to around 96 or 97. But the second four hours are marked by periods of rapid eye movement (REM) sleep, a phenomenon in which most of our dreams take place, along with a host of physical changes.
If you Google around for this question, you’ll end up with a bunch of theories about blankets simulating the warm, enclosed feeling we had in the womb. There could be some element of theoretical protection or security imbued by the blanket, which might be another bit of conditioning, but Hoagland thinks the womb comparison is pretty unlikely. “I’m very suspicious of anyone who implies that this goes back to the feeling of being in the womb,” she says. “I think that’s very far-fetched.”
Another possible reason is that blankets are soft and feel good. I could not find any studies that examine the question of whether people like blankets because they’re soft and feel good, so this may remain a great unanswered question.