If you‘re one of those people who sleep 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 behavioural, 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 inherited from father to son Instead of covering up with a fluffy blanket, 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 somesleep experts recommend taking a bath or shower before bed, since your body will naturally cool of afterward, signalling to your body that it’s time to go to sleep. Later in the night, thought during REM sleep, your body cant’s 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 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. There are more simple psychological reasons to cover up too. When you’re baby, your parents put blankets on you when you sleep, so you’re conditioned throughout your early years to associate blankets with bedtime Above all, maybe we just all want to be swaddled forever. Doesn’t that sound nice?