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In discussing sleep, we throw around a host of expressions handed down from history. Why, for example, do we tell people to sleep tight? Or why should we let sleeping dogs lie? We put on our etymological hats to find the origins of these five common phrases. Study them and you just might impress some friends or gain an edge in your next round of bar trivia. 

“Sleep Tight”

There are a few different origin-stories floating around about this often-used phrase, one of which theorizes that it first popped up in the 1700s. You see, the modern bed we know has only been around for about a century. Before then, people often slept on mattresses supported by criss-crossing ropes.  Loose ropes left people with saggy sleeping surfaces, so tightness was critical to a good night’s sleep.

It's also theorized that the phrase was a tongue-in-cheek taunt regarding bed bugs. Mattresses weren’t always made of memory foam, and sleepers commonly found the blood-sucking critters strutting near the straw stuffing. There were proper defenses in order, but any errant sheet or untucked corner could lead to a night-feasting. Hence the pre-bed saying of “sleep tight, don’t let the bedbugs bite.”

According to the Oxford English Dictionary, however, “sleep tight” may simply mean “sleep well,” as people once used “soundly” and “tightly” interchangeably.

“ZZZs”

The use of “Z” to indicate snoring dates back to at least 1918. While some people claim the letter was chosen for its acoustic similarity to snoring, that’s not quite true. You see, it all had to do with its shape. “Z” visually represents the motion of sawing wood — and that’s the sound that someone associated with snoring. Over time, the  Z-bubble, most often seen in comics, has broadened in meaning to symbolize the general act of sleeping.

“The Wrong Side of the Bed”

This common idiom for waking up in a bad mood comes from Augustus Caesar and his fellow superstitious Romans, who made sure not to get up on the left (sinister) side of the bed for fear it would lead to bad omens.

“Three-dog night”

The phrase (from which the 1960s rock trio took its name) denotes an evening so bitterly cold that one needs the warmth of three dogs — one for a pillow, one at the middle and a third at the feet  — to sleep through the night. People have traced its geographic origins to a number of notably frigid locales, including the Australian Outback and Western Canada.

Linguists pointed to a passage from a 1910 history book that may have introduced the concept of measuring sub-zero temperatures by the number of dogs needed for nocturnal insulation: “To sleep in the open at 45 to 50 degrees below zero, with two or three dogs crouching on one's person for the sake of the heat they emit, is scarcely episcopal.”

The phrase itself may have first appeared in English in 1957. A story in the Stuebenville Herald, an Ohio newspaper,  said,  "A chilly night is known as a 'three-dog night.' A five-dog night' is really cold."

Let Sleeping Dogs Lie”

When we think people should refrain from instigating a situation, we might tell them to let sleeping dogs lie. It's thought that the expression goes back to the Bible but the phrase didn’t appear in the good book verbatim. In the Book of Proverbs 26:17 it was urged: “He that passes by, and meddles with strife belonging not to him, is like one that takes a dog by the ear.” By the 1700s, however, the phrase evolved to its present form and entered circulation, as the British Prime Minister Sir Robert Walpole was quoted using it more than once.