Is "I slept like a rock" a simile?

"I slept like a rock" is a simile. "I was a rock last night" is a metaphor. The single word like is the whole difference, and it's also the entire test: when like or as is doing the comparing, you have a simile. The reason this particular sentence trips people up is that "slept like a rock" is a dead simile, a comparison worn so flat by reuse that most readers no longer register it as a figure at all, which is exactly the conditions under which a simile starts to feel like something else.

What makes it a simile and not a metaphor?

A simile (a trope of comparison using like or as) sets two things next to each other and keeps them separate. A metaphor (a trope of substitution) calls one thing the other.

In "I slept like a rock," like compares the manner of sleeping to the stillness of a rock. The sleeper is one thing; the rock is another; the figure holds them apart and points at the resemblance between them. The sleeper does not become the rock. That separation is the diagnostic feature of a simile.

The metaphor version of the same idea is "I was a rock last night." There is no like and no as. The sleeper is the rock, identified with it rather than compared to it. Same vehicle, different figure.

The test in practice fits on one line. If like or as is doing the comparing, it's a simile. If the sentence identifies one thing with another instead, it's a metaphor.

Phrase	Figure	Why
"I slept like a rock"	Simile	Uses like; sleeper and rock stay distinct.
"I was a rock last night"	Metaphor	No like or as; the sleeper is identified with the rock.

Why does it feel like it might be a metaphor?

Because "sleep like a rock" is a dead simile, a figure so worn by repeated use that the comparison stops registering. The phrase has shifted in the ear from "here is a vivid comparison" to "here is the standard way to say I slept hard." The comparison still works on the page; it just no longer surprises anyone, and an unsurprising comparison can feel less like a figure than a fixed expression.

The same flattening happens on the metaphor side. "Leg of the table," "head of the line," "foot of the bed" are all metaphors that lost their figural charge generations ago, which is exactly what a dead metaphor is. Similes can die the same way, and "slept like a rock" is a textbook case.

Dead doesn't mean misclassified. The figure is still a simile mechanically; the like is still doing the same syntactic and semantic work it does in a fresh comparison. Classification follows the mechanism, not the temperature.

How does the test work on similar sleeping phrases?

Run the like/as check on the surrounding family and the picture clears up quickly:

• "Sleep like a log." Simile. Like is doing the comparing; the sleeper is held next to the log, not identified with it.
• "Sleep like a baby." Simile. Same structure, same test, same verdict.
• "Dead to the world." Metaphor. There is no like or as. The sleeper is identified with dead, not compared to it.
• "Out cold." Not a figure of comparison at all. It's an idiom that intensifies unconscious; closer to hyperbole than to simile or metaphor.

The check is durable. The next sleeping phrase you meet, run like or as through it, and you have your answer.

Is it still a simile if it's a cliché?

Yes. The classification is independent of freshness. A simile is defined by its mechanism (comparison via like or as), not by how original the comparison happens to be. A first-draft simile and a thousand-year-old one are both similes if the structural test holds.

The practical question is a different one. If "slept like a rock" reads flat in a draft, the fix isn't to reach for a new label. It's to pick a less worn comparison, or to drop the figure entirely and let plain language do the work. The figure is correctly identified; it just isn't earning its place in the sentence anymore.