I guess it´s the same with pigs. Most of my pigs sleep with their eyes open when I´m doing stuff around, but you can tell they are still somewhat alert. When they are in a deep sleep, their eyes are closed.
One of my pigs (he died last summer) used to freak me out. He always slept like a dog, all curled up with the chin resting near the back legs. Sometimes, he´d roll over a bit, his eyes would roll in the back of his head and his tongue hung out to the side of his mouth. Do I need to tell you how freaked out I was when I turned around and he was like that? The poor pig probably died from a heart attack after I woke him up screaming and shaking him so many times.
- I GAVE, dammit!
It seems my more relaxed pigs close their eyes. Some of my more timid pigs I´ve never caught before.