If you‘re 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 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 if they were very wealthy.