(SUBSTANCE) [noun] [U] [informal] - a yellowish green substance sometimes found in the corners of the eyes after sleepingYou haven't washed properly, Lola, you've still got sleep in your eyes.(figurative) I was just rubbing/wiping the sleep out of my eyes (= waking) when you called.
SLEEP
Meaning of SLEEP in English
Cambridge English vocab. Кембриджский английский словарь. 2012