Why do we rub our eyes after waking up?

I don't rub my eyes when I wake up but I notice my little one's do this and have always figured it was to take the sleep out of their eyes so they could see better or just because when you first wake up everything seems so much brighter and you need your eyes to adjust a bit to the light and this seems to help .

I don't! Years ago I was told doing this can cause eye problems and also promote saggy eyes later in life from stretching the skin area around the eyes. I gently remove "sleepies" from around my eye area and the minute I get up I rinse my eyes with cool water.