Sleep with one eye open

Meaning

Be alert even while supposedly resting.

Usage

Used when someone remains cautious in risky situations.

Origin & History

Origins and dates here are approximate summaries. Many idioms have disputed or unknown origins, and not every date has been checked against a historical dictionary, so treat specific dates and stories as a starting point rather than a citation.

Imagery of half-open eyes to detect danger.

Used in wartime and frontier tales to describe vigilance.