Sleep with one eye open
Meaning
To remain extremely vigilant and cautious, even while resting, due to a sense of danger or unease.
Origin
The vivid imagery behind "sleep with one eye open" comes directly from the animal kingdom, where many species, particularly prey animals and marine mammals like ducks and dolphins, literally possess the ability to do just that. Through a remarkable evolutionary adaptation called unihemispheric sleep, one half of their brain can rest deeply while the other half remains awake, allowing them to keep one eye open to watch for predators or navigate their environment. Humans, lacking this exact physiological trick, embraced the phrase as a powerful metaphor. It perfectly encapsulates the feeling of intense vigilance and unease, where one's guard is never truly down, even during moments meant for repose, perpetually scanning the horizon for any hint of danger.
Examples
- After the recent break-ins in the neighborhood, many residents started to sleep with one eye open, constantly listening for unusual sounds.
- Even on vacation, the CEO admitted he had to sleep with one eye open, always aware of potential market shifts and competitive threats.