Sentence:  The part of the eyelid that is the location of the lacrimal caruncle, which produces rheum or "<WSD>sleep</WSD>," the gritty substance often present when awakening. 
Meaning of the word marked with <WSD> tags:  Rheum, crusty or gummy discharge found in the corner of the eyes after waking, whether real or a figurative objectification of sleep (in the sense of "reduced consciousness").