Since the lower eyelid area has very thin skin and net of fine blood vessels, it is an area particularly vulnerable to bruising.
So, a simple rubbing during sleep can result in a bruise.
This is more common if your husband is taking aspirin, motrin or other drugs that are considered blood thinners.
This can happen due to a high blood pressure, too.
He may visit his physician and ask for certain blood test for detecting possible clotting problems, regular blood count test and test for thyroid hormones.

Best wishes!
Marija