Yes, they can. Sometimes they will keep their eyes closed if they are very relaxed. I have found temperament may come into play... I have some very mellow guinea pigs who often sleep with eyes closed, but also had more high-strung guinea pigs who very seldom closed their eyes all the way- Sundae would sometimes have her eyes at half-mast, but I never once saw Frenzy sleep with her eyes shut in her entire life! But if they are laying down, looking comfy, they are likely dozing, even if their eyes are open.