What would cause extremely dry eyes at night only?

Lids open at night. A common cause of night-time dryness is the eyes drifting open during sleep. The exposed portion of the eye dries out, producing often significant symptoms if we awake during the night or first thing in the morning. Using an otc lubricating ointment in the eyes at bedtime is helpful in theses cases, since the ointment coats the surface of the eye, preventing drying out if it should drift open.
Dry Eye Syndrome. Whether it is caused by allergies, environmental dryness or a serious medical condition like conjunctivitis or sjogren's syndrome, eye dryness is the result of reduced tear gland activity. When the body sleeps, its functions slow and rest. If the eye's tear drops do not properly lubricate it during the day, the eye will not gain relief at night and become more dry and irritated.