Q: When I go to bed without removing my contact lenses, I sleep in my contacts. Or so I say, even though the reverse is true: my contacts are in me when I sleep. What say you?
A: The preposition “in” has been used to mean “wearing” since Anglo-Saxon days.
The earliest example in the Oxford English Dictionary, from an Old English translation of Exodus, refers to mourners in blacum reafum (in black robes).
You’re right, though, that when “in” is used this way we’re usually in clothing of one sort or another (a dress, a suit, a dinner jacket, and so on).
However, we sometimes use “in” loosely to mean “wearing” when we’re not literally inside things—or at least not very far inside them. For example, we say we’re “in curlers” or “in a wig” or “in a beret.”
More important, the expression “in my contacts” is an idiom, and idioms don’t always make sense on a literal level. We’ve written often on our blog about idioms, including a post a few years ago entitled “Can an idiom make sense?”
As we said then, an idiom is a peculiarity of language—an expression or some characteristic of speech that’s peculiar to a language, a region, a dialect, or a group of people.
Sometimes an idiom doesn’t make literal sense (“it’s raining cats and dogs,” or “he reached for the stars”). At other times it’s grammatically unusual or doesn’t parse (“I could care less,” “that dress isn’t you”).
An idiom can also be a specialized language or vocabulary used among a particular group—like doctors or journalists. Or it can be a particular regional or dialectal speech pattern.
By the way, the term “contact lens” may be a lot older than you imagine. The two earliest examples in the OED are from an 1888 issue of Archives of Ophthalmology. Here’s one citation:
“The ‘contact-lens’ consists of a thin glass shell, bounded by concentric and parallel spherical segments.”
The first example in the dictionary for the term “contacts” used in place of “contact lenses” dates from 1961, but we’ll end with this more recent OED citation:
“I can’t wear glasses because it hurts my nose. I can’t wear contacts because it hurts my nerves” (from Money: A Suicide Note, an 1984 novel by Martin Amis).