I've once heard a person make a reference to another person as sleeping with their eyes open. I wonder if there is another way to take this expression other than literally. Thanks.
Sleeping with one eye open may be an expression meaning to sleep lightly, aware of what is going on around, but apparently it actually happens: it is a condition known as lagophthalmos.
Apparently, some people do sleep with both eyes open.
It is not meant to be taken literally. It essentially means to be careful, or watchful. The phrase is generally context dependent.
E.g. "I don't trust her, you should sleep with your eyes/one eye open" would mean to wary of her.