A nightshirt is a long, loose shirt designed to be worn to bed. Picture something like an extra-long t-shirt or button-up shirt that hangs down past your knees, made from soft, comfortable fabric like cotton or flannel.

Before pajamas became popular in the early 1900s, nightshirts were what most people wore to sleep. Men, women, and children all had nightshirts, though the styles looked slightly different for each. They were practical: easy to move around in, simple to wash, and comfortable in both warm and cool weather.

You've probably seen nightshirts in old paintings or classic stories. Ebenezer Scrooge wears one in A Christmas Carol, and nightshirts appear in countless illustrations of people from the 1800s getting ready for bed or being woken up in the middle of the night.

Today, most people wear pajama sets instead, but nightshirts haven't disappeared. Some people still prefer them because they're loose and don't bunch up when you roll over in your sleep. They're especially popular in flannel during winter months. The style has remained essentially unchanged for centuries: a simple, comfortable shirt long enough to keep you covered while you sleep.