Bring the attention at the eyes inwards to any focal point you wish, be it Dan Tian, Zhong Tian (Middle field/heart chakra center),
Pineal Gland, or Crown, or all of them. If closed, it is easier to fall asleep, and also see images in the mind. That makes it easier to follow them, which you don't want to do. So, if you keep your eyes closed, know that what you see is just passing, and don't follow them.
Eyes open causes the eyes to dry up, and it sucks. You don't want that. However, opened eyes cultivate an attentive and active mind (also meaning energy here). Eyes half closed is good, doesn't let your eyes dry up, but doesn't allow you to nod off. If you can stay awake with eyes closed, go for it. Either way, any position of the eyes is fine just as long as you grasp the attention from the eyes, and direct it inwards. This goes for any of the other senses.
Hope this helps