It's very normal for guinea pigs to sleep with their eyes open. It's actually very unusual for them to close their eyes while sleeping. Guinea pigs are prey animals and their wild ancestors would typically sleep with eyes open and their feet under them, ready to run away from any sudden movement. If your piggies ever sleep with any of their feet exposed or if they actually lay their head down, take it as a compliment. They feel very safe in the home you've provided for them.