The medical term is rheum, and it's just a natural buildup of dried tears, mucus, and dirt. A possible sentence might be: I didn't sleep enough for rheum to form in my eyes. However, that sentence is really awkward and most people (non healthcare professionals) might not have any idea what you're talking about.
New words, one handy idiom, and a 2-minute quiz — delivered to your inbox to keep your streak alive.