If you’re one of those people who sleeps with a sheet even on the hottest of nights, you’re not alone. Plenty of people can’t fall asleep if they aren’t covered with something, even if it’s the lightest of blankets. Why? Dan Nosowitz at Atlas Obscura reports that it’s both physiological and behavioral, and may have a component of simple conditioning.

Surprisingly, he found, sleeping with blankets is a relatively new phenomenon. Historically, blankets were expensive. Through the Middle Ages, Europeans only owned blankets if they were very wealthy. They were so precious, in fact, that bedding was passed down in people’s wills. Instead of snuggling up with a fluffy duvet, most people slept in the same bed as the rest of the household, farm animals included, to keep warm. But as fabric became cheaper and blankets more accessible, they became more commonplace household items. Now, even in tropical places, many people cover themselves with at least something during the night, with the exception of some nomadic cultures near the equator.

Part of the reason is that the body really does need extra warmth at night. Your body’s internal temperature begins to cool down before you go to bed. That’s one reason why some sleep experts recommend taking a bath or shower before bed, since your body will naturally cool off afterward, signaling to your body that it’s time to drift off. (Sticking one foot outside the covers can help, too.) Later in the night, though, that cooling off gets less pleasant and more, well, cold. During REM sleep, your body can’t regulate its own temperature. And for the most part, people tend to be in the REM stage of sleep right around dawn, when temperatures are the coldest. So we naturally learn that even if it’s pretty hot when we go to bed, we’ll wake up shivering at 4 a.m. if we don’t have a blanket.

And then there’s the neurological reason: Weighted blankets have been found to decrease anxiety and stress, because gentle pressure can stimulate serotonin production. Serotonin has been found to help modulate sleep regulation, which is part of the reason that depression and insomnia are linked—when you’re depressed, your serotonin levels are low.

There are more simple psychological reasons to cover up, too. When you’re a baby, your parents put blankets on you when you sleep, so you’re conditioned throughout your early years to associate blankies with bedtime. Above all, maybe we just all want to be swaddled forever. Doesn't that sound nice?

[h/t Atlas Obscura]