Tears contain an important hormone that kills bacteria. Closing eyelids during sleep allows a prolonged contact between the cornea and tears killing resistant bacteria in the process. The action of the tears during blinking is not sufficient to completely get rid of the foreign bodies in the eyes. Closing eyelids gives the tears a capacity to accumulate forming a film of moisture above the cornea and acqeous humour. This has many important uses. It allows dust particles to dissolve and be eradicated when the tears flow out of the eyelids. This partially explains the residue that is seen on the cheeks when people wake up in the morning in addition, of course, to the sodium and potassium salts in the tears. The other importance of the film of tears accumulated on the eye is to help keep the acqeous humour moist. This has a great effect on how the lens receive light and also on the overall refractive index of the eye as system.
