Every once in a while my eyes feel like they have sand in them. what causes this?

Dryness: Most likely your eyes have some degree of dryness. This is sensitive to your bodily hydration, atmospheric moisture and history of eye or systemic disease. If minor and does not affect the vision nor cause eye redness, you can apply over the counter lubricating eyedrops. If not minor, you should consult with your ophthalmologist.

Dry eyes most common: Other possibilities are erosions or sleeping with eyes partially open. Try lubricants and if not improving see you eye doc.